Add a --debug-mode option to aapt.

When passed (with no needed parameters) to the aapt command line,
aapt will insert debuggable=true in the application node of the manifest
automatically.

This is to be used by the SDK tools to make true "debug" builds
that require no code/file change.

Change-Id: I909759caef499a91d10cc9a0902c6448c87e75ef