Android: Fix app compatibility with "master channel"

UPDATE

Due to the dependency with the aqwert/flutter_platform_widgets#90 issue, we cannot currently support master. I will update the README.md accordingly.

---

Currently the app won't build with the Flutter master channel, but it still works with stable or beta.

Flutter Doctor:

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel master, v1.10.6-pre.19, on Mac OS X 10.15 19A583, locale
    en-DE)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
[✓] Xcode - develop for iOS and macOS (Xcode 11.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 3.5)
[✓] IntelliJ IDEA Ultimate Edition (version 2019.2.1)
[✓] Connected device (3 available)

• No issues found!

Error logs

/Users/miquel/dev/projects/timy-messenger/android/app/src/debug/AndroidManifest.xml:22:18-91 Error:
	Attribute application@appComponentFactory value=(android.support.v4.app.CoreComponentFactory) from [com.android.support:support-compat:28.0.0] AndroidManifest.xml:22:18-91
	is also present at [androidx.core:core:1.0.2] AndroidManifest.xml:22:18-86 value=(androidx.core.app.CoreComponentFactory).
	Suggestion: add 'tools:replace="android:appComponentFactory"' to <application> element at AndroidManifest.xml:9:5-36:19 to override.

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:processDebugManifest'.
> Manifest merger failed : Attribute application@appComponentFactory value=(android.support.v4.app.CoreComponentFactory) from [com.android.support:support-compat:28.0.0] AndroidManifest.xml:22:18-91
  	is also present at [androidx.core:core:1.0.2] AndroidManifest.xml:22:18-86 value=(androidx.core.app.CoreComponentFactory).
  	Suggestion: add 'tools:replace="android:appComponentFactory"' to <application> element at AndroidManifest.xml:9:5-36:19 to override.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 16s
AndroidX incompatibilities may have caused this build to fail. See https://goo.gl/CP92wY.
Finished with error: Gradle task assembleDebug failed with exit code 1

TODO:

• Update Android project to fix issue.