Build overlay packages just like regular packages.

Previously, building overlay packages required passing aapt its -o flag.
This commit decouples the idmap generation code from the effects of the
-o flag.

Since this commit renders the -o flag obsolete, support for the flag was
removed from aapt as well.

Change-Id: Ied2e0ab8cb800e49623f0a2044b06cd4935473d5