Add tests for layout overlaying

Add tests to ensure that findViewbyId retrieves the correct view when
an id is overlaid in a RRO.

Also fixes a bug that made strings hardcoded in the overlays.xml
file unable to be retrieved from the string pool.

Bug: 135943783
Test: atest OverlayDeviceTests
Change-Id: I2bf03f151cb696d28f6bb9018eb319af29ba48f4