Android: Say hello to gradle!

Switching from Ant to Gradle brings lots of advantages:
 - it is way faster when rebuilding (25-50% faster than ant).
 - it enables first class Android Studio integration.
 - adding Android Extras libs (e.g. Google Play services, OBB, etc.) to
   your project is now painless.

[ChangeLog][Android] Added Gradle support to build the APK.

Change-Id: Iee492954f8ffb2c22e6ab14a8a25faf644de9a51
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/android/androidmanager.h

@@ -96,6 +96,8 @@ public:
     static bool checkCertificatePassword(const QString &keystorePath, const QString &keystorePasswd, const QString &alias, const QString &certificatePasswd);
     static bool checkForQt51Files(Utils::FileName fileName);
     static AndroidQtSupport *androidQtSupport(ProjectExplorer::Target *target);
+    static bool useGradle(ProjectExplorer::Target *target);
+    static bool updateGradleProperties(ProjectExplorer::Target *target);
 };
 
 } // namespace Android