Remove prefix from build_gradle_dependencies warning

[rmartin16]

Changes

• The new warning for build_gradle_dependencies caught me off guard with the prefix...so, I removed it.
• Logging these boxed warnings without a prefix is more in line with other uses; feel free to close if you disagree.

Before:

[androidrequests] Generating application template...

[androidrequests] 
[androidrequests] *************************************************************************
[androidrequests] ** WARNING: App does not define build_gradle_dependencies              **
[androidrequests] *************************************************************************
[androidrequests] 
[androidrequests]     The Android configuration for this app does not contain a
[androidrequests]     `build_gradle_dependencies` definition. Briefcase will use a default
[androidrequests]     value of:
[androidrequests] 
[androidrequests]         build_gradle_dependencies = [
[androidrequests]             "androidx.appcompat:appcompat:1.0.2",
[androidrequests]             "androidx.constraintlayout:constraintlayout:1.1.3",
[androidrequests]             "androidx.swiperefreshlayout:swiperefreshlayout:1.1.0",
[androidrequests]         ]
[androidrequests] 
[androidrequests]     You should add this definition to the Android configuration
[androidrequests]     of your project's pyproject.toml file. See:
[androidrequests] 
[androidrequests]         https://briefcase.readthedocs.io/en/stable/reference/platforms/android.html#build_gradle-dependencies
[androidrequests] 
[androidrequests]     for more information.
[androidrequests] 
[androidrequests] *************************************************************************
Using app template: https://github.com/beeware/briefcase-android-gradle-template.git, branch v0.3.17

After:

[androidrequests] Generating application template...

*************************************************************************
** WARNING: App does not define build_gradle_dependencies              **
*************************************************************************

    The Android configuration for this app does not contain a
    `build_gradle_dependencies` definition. Briefcase will use a default
    value of:

        build_gradle_dependencies = [
            "androidx.appcompat:appcompat:1.0.2",
            "androidx.constraintlayout:constraintlayout:1.1.3",
            "androidx.swiperefreshlayout:swiperefreshlayout:1.1.0",
        ]

    You should add this definition to the Android configuration
    of your project's pyproject.toml file. See:

        https://briefcase.readthedocs.io/en/stable/reference/platforms/android.html#build_gradle-dependencies

    for more information.

*************************************************************************

Using app template: https://github.com/beeware/briefcase-android-gradle-template.git, branch v0.3.17

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Agreed this is more readable. +1