Add tests for RRO of APK-in-APEX.

Add tests to ensure that 1) an APK-in-APEX can overlay a /system APK
2) an APK-in-APEX can be overlaid be a /system APK. This is done
by installing the overlay APK or the target APK inside a test APEX.

Bug: 147731966
Test: atest OverlayRemountedTest
Change-Id: Id2c331f124994f55cd936b98ec2f27c9d3265424