Android: Don't let the user choose a stupid ANDROID_PACKAGE_SOURCE_DIR

On creating a AndroidManifest.xml we ask the user where to put the file,
which as a sideeffect also sets ANDROID_PACKAGE_DIR.

Since we copy everthing from ANDROID_PACKAGE_SOURCE_DIR into the build
directory, using the project's source directory is not desireable.

We now show a big error if the user tries that.

Task-number: QTCREATORBUG-11708
Change-Id: I7fade3efac0b7466cad8e83a92c2115d0a70c683
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>