RRO: remove ability to overlay assets

Target packages use <overlayable> to opt-in to RRO. While this works for
resources, it does not support assets. For security reasons, remove the
ability to overlay assets via RRO for now: a future change may extend
<overlayable> to include assets.

Test: atest OverlayDeviceTests
Bug: 120867881
Change-Id: Ie0b6897f0586ea8b9d75f357e216904e25d4dc7b
8 files changed