Implement custom build step for ant to generate metadata

[atanasovg]

The runtime expects a pre-built metadata as part of the application package. This metadata is currently manually updated in the test application. Additionally, at CLI level, we run the ant build twice to ensure proper metadata and then to embed it in the application package.

To resolve this, we should:

• Reference the metadata generator in the test application.
• Add a custom build step for the -post-compile hook in ant.
• Run the metadata generator on the already built source plus the minimum targeted Android API level.
• Use the <uses-sdk android:minSdkVersion="17" android:targetSdkVersion="21" /> information available in the AndroidManifest.xml (we should be able to retrieve this from ant's variables) to get the proper Android.jar version.
• Put the output of this custom build step in the assets/metadata folder and let ant continue with the package step.

Once this is done, we will use the same logic at CLI level to ensure the proper metadata, depending on the minimum targeted Android API level.