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: I6f0a7af7b7d51f26bb0ec012e6f142a6060b8618