Expose "android-extra-plugins" androiddeployqt feature to qmake

Makes it possible to use "android-extra-plugins" from qmake through the
ANDROID_EXTRA_PLUGINS variable.

Change-Id: I7c67e9f104e5397e094afff730efccb91949caa2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>