Fix Qt for Android build with recent SDK bundle

In the recent revisions of the Android SDK bundle, the dx
tool has moved from platform-tools/ and into
build-tools/<revision-number> where revision-number is 17.0.0 at
the moment. To enable building on these SDKs, we add
detection for the dx tool and an environment variable which
can be set to override the revision number to provide a way
to build it against future revisions as well.

Task-number: QTBUG-31199
Change-Id: I0d6a22163dc2e50f7a81cd3fe8f3d53c6335aaee
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>