[Android] Remove deprecated APIs and fix for a few found issues

[vbuberen]

This PR mainly addresses and closes #32

Changes:

• Replaced deprecated middleware() from Segment Android with suggested useSourceMiddleware()
• Removed registerWith() as it is also deprecated and was needed only when Flutter Embedding V1 was used, while the plugin already switched to V2, so register will happen automatically.
• A few fixes for lint warnings, like adding correct annotations, switch to lambda in userSourceMiddleware(), etc.
• Add a teardown for methodChannel in onDetachFromEngine as it should be done.
• Bumped Android Gradle plugin to 7.2.1 as it was a hotfix for 7.2.0
• Updated example to not use jcenter.

[danielgomezrico]

@vbuberen I tried and on compilation it fails with:

/Users/daniel/.pub-cache/git/flutter-segment-95ad9e4b3158442f3dd78f6d0dade7b03b8a2875/android/src/main/java/com/example/flutter_segment/FlutterSegmentPlugin.java:90: error: lambda expressions are not supported in -source 7
              chain -> {
                    ^
  (use -source 8 or higher to enable lambda expressions)

[vbuberen]

@vbuberen I tried and on compilation it fails with:

/Users/daniel/.pub-cache/git/flutter-segment-95ad9e4b3158442f3dd78f6d0dade7b03b8a2875/android/src/main/java/com/example/flutter_segment/FlutterSegmentPlugin.java:90: error: lambda expressions are not supported in -source 7
              chain -> {
                    ^
  (use -source 8 or higher to enable lambda expressions)

Looks like I needed to explicitly specify in build.gradle the following block:

compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

In latest Android Studio versions it is not an issue as project builds fine even without that part I mentioned above.