CMake/Android: enable Gradle for building Qt JARs

Add Gradle project structure for JAR builds which allows
CMake to run a Gradle build and also enables such code to
be managed from IDEs easily.

The default Android platform Jar include won't be needed
because it's handled by Gradle, and the main QtAndroid.jar
include is added automatically by qt_internal_add_jar().

This also update gitignore with Gradle, JDTLS and IntelliJ
ignores. It also removes files that might have been used
exclusively for javac and some that are not needed for
neither javac nor Gradle.

Task-number: QTBUG-132915
Change-Id: I8afe450579c6b5eb769d3b56b7e4a15f0addf393
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>