Fix Gradle setup in android folder

[friederbluemle]

build.gradle in android/ did not specify the Android Gradle plugin, making it impossible to build or open the folder in isolation.

Note that the plugin is only required when opening the project stand-alone (not when used as a dependency from an app project). Therefore, it has been wrapped in a project == rootProject statement).

In addition to that, several other related issues were present in that folder, all of which have been fixed in this PR:

• android.iml file should not be in source control (already part of .gitignore, removed file)
• repositories was missing the relative path to the react-native dependency, incorrectly pulling an ancient version from mvncentral
• The react-native dependency was declared as api - This is not necessary, since app project bring their own version of react-native. Changed it to the default implementation, since there is no need to expose this downstream
• Removed a redundant dependency on appcompat (already provided by react-native)

Fixes #669

Test Plan:

Use Android Studio and open android/. ("Build" ->) "Make Project".

or

1. Generate Gradle wrapper on command line: gradle wrapper --gradle-version 5.4.1 --distribution-type all
2. ./gradlew build

[janicduplessis]

Looks good, thanks for cleaning this up!