Search for RRO both in SKU subdir and in vendor/overlay

Currently, if the "ro.boot.vendor.overlay.subdir" property is set, we
only search in the subdir. It seems more powerful to search both in
there and in the standard /vendor/overlay directory.

Also, RRO packages have the "priority" attribute in the <overlay>
attribute in AndroidManifest.xml which serves for disambiguation.

Bug: 32202741
Change-Id: I7acff1b7f0e153830924047349f003295648a53b