(android) build_apprtc and build_debug_apprtc can take a revision number

This makes it easier to select which revision to build. For example:

  build_apprtc 6783 # builds r6783
  build_apprtc # still builds the latest revision

Inspired by https://code.google.com/p/webrtc/issues/detail?id=3622#c12
Merge pull request #9 from joseph-onsip/revarg

(android) build_apprtc and build_debug_apprtc can take a revision number