CMake isn't exposing an aab target for Android projects

CMake isn't adding a custom target for building aab packages, only apk custom targets exist. This will save us from having to call androiddeployqt manually with --aab flag.

We should add both target specific and global aab custom targets, similar to the apk ones.

In contrast to the apk targets, the proposal is to make aab targets not be part of ALL target, so running just 'ninja' won't create aabs, but it will create apks.
The reason is that aabs are mainly used for store publishing and the default built aabs are not signed, so there's no need to use extra build space until the user knows they want to publish the aabs, in which case they can do that explicitly.

Qt 5 CMake code can be checked for the aab creation command line flags.