AAPT2: Support compiling a res/ directory and output to zip

This allows us to compile an entire directory and output to a single
file. This is important to support generated resources in the make
build, since we may not know what resources get generated.

The link step will accept the zip and read the contents of it as if they
were passed in on the command line.

Change-Id: If1a51b0abe772350c24074353eb4989953c2e0cb