[monodroid] don't probe for .exe files

[jonathanpeppers]

I was noticing a lot of logging like this:

11-25 13:14:59.430 13111 13111 I monodroid-assembly: open_from_update_dir: trying to open assembly: /data/user/0/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll
11-25 13:14:59.431 13111 13111 I monodroid-assembly: open_from_update_dir: trying to open assembly: /storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll
11-25 13:14:59.431 13111 13111 I monodroid-assembly: open_from_update_dir: trying to open assembly: /data/user/0/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll.dll
11-25 13:14:59.431 13111 13111 I monodroid-assembly: open_from_update_dir: trying to open assembly: /storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll.dll
11-25 13:14:59.431 13111 13111 I monodroid-assembly: open_from_update_dir: trying to open assembly: /data/user/0/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll.exe
11-25 13:14:59.431 13111 13111 I monodroid-assembly: open_from_update_dir: trying to open assembly: /storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll.exe
11-25 13:14:59.431 13111 13111 I monodroid-assembly: open_from_bundles: looking for bundled name: 'mscorlib.dll'
11-25 13:14:59.431 13111 13111 D Mono    : Image addref mscorlib[0x7f74876f3f00] (asmctx DEFAULT) -> mscorlib.dll[0x7f747fb32000]: 2

A couple things we could do here is:

1. Don't probe for .exe files at all. We don't even consider .exe
   files at build time anymore since e390702.
2. Check if the name ends with .dll to decide if .dll should be
   appended when probing.

This change is needed in both open_from_update_dir and
open_from_bundles.

Now the probing is cut down to:

11-25 13:24:42.060 16492 16492 I monodroid-assembly: open_from_update_dir: trying to open assembly: /data/user/0/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll
11-25 13:24:42.060 16492 16492 I monodroid-assembly: open_from_update_dir: trying to open assembly: /storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/mscorlib.dll
11-25 13:24:42.060 16492 16492 I monodroid-assembly: open_from_bundles: looking for bundled name: 'mscorlib.dll'
11-25 13:24:42.063 16492 16492 D Mono    : Image addref mscorlib[0x7f74876f2f80] (asmctx DEFAULT) -> mscorlib.dll[0x7f747fb33000]: 2

Results

Using the samples\HelloWorld app in this repo, on a Pixel 3 XL:

Before:
11-25 12:00:42.863 31251 31251 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::704288
11-25 12:00:44.997 31306 31306 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::558089
11-25 12:00:47.137 31359 31359 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::936683
11-25 12:00:49.201 31412 31412 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:164::536735
11-25 12:00:51.330 31464 31464 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:166::357204
After:
11-25 12:08:34.160 32331 32331 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::597673
11-25 12:08:37.022 32384 32384 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:164::106162
11-25 12:08:39.237 32439 32439 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:164::997516
11-25 12:08:41.841 32490 32490 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:164::931683
11-25 12:08:44.731 32551 32551 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:164::95381

My guess is this is a 1-2ms improvement on this device.

So then I tried an x86 emulator using HAXM:

Before:
11-25 13:19:06.679 14703 14703 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:501::511300
11-25 13:19:09.191 14753 14753 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:545::571100
11-25 13:19:11.902 14827 14827 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:555::89900
11-25 13:19:15.503 14880 14880 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:541::920000
11-25 13:19:18.377 14981 14981 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:509::316700
After:
11-25 13:27:24.287 17169 17169 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:472::543900
11-25 13:27:26.777 17216 17216 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:446::208700
11-25 13:27:29.369 17265 17265 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:469::216400
11-25 13:27:31.977 17313 17313 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:420::160500
11-25 13:27:34.700 17357 17357 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:437::225200

Here the improvement might be more like 50-75ms. I will probably focus
on timing the HAXM emulator from now on, since I am focusing on
improving the developer loop here.