Fix handling android dependency xml.

Run readAndroidDependencyXml only once for every Qt library.

Task-number: QTBUG-34078

Change-Id: Ifc2c18f23485e2ced20b0ce3f151a21cb1a0069a
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>