Add single crunch command to aapt.

Previously the crunch command would work on a full res folder
and output a full res folder (with only the drawables). This
was only used in the SDK.

The incremental logic is moved to the SDK build system so we
change the crunch command (or rather add a new one) to only
crunch a single file.

Change-Id: I635ee3e871d035b9db2fb593802d914e48241abf