OMS: add tests for the OMS transactional API

Add test cases for the new OMS transactional API.

Also, migrate the existing OverlayDeviceTests from the overlay manager's
shell interface to OverlayManager and transactions.

Bug: 119916381
Test: atest OverlayDeviceTests
Change-Id: Ie5344c53dbd470c0eb3143798d27ac490096b34e