Add test target to f/b/packages/overlays.

- Added IconPackOverlayTest to ensure that there icon pack overlays do not
get out of sync with the underlying drawables in each apk.
- Remove overlayable_icons_test.xml from each app since this test makes
them no longer necessary
- Add apcts target OverlayTests.apk - TODO add this to presubmit config
for settings and systemui

Bug: 134566901
Test: atest IconPackOverlayTest
Change-Id: I99243db4839ac41dad01abf9f83ebda757dbbb77