This removes an API < 10 code path, which should speed up all
`JNIEnv.StartCreateInstance` / `FinishCreateInstance` calls by a very
small amount. They no longer check a `AllocObjectSupported` field,
which is now always `true`.

This also removes the need to pass in `androidSdkVersion` /
`android_api_level` from the native side at startup.

We can also remove the `_monodroid_get_android_api_level()` p/invoke,
which is unused.