OverlayDeviceTests: don't rely on Thread#sleep

When a request to enable an overlay is received by the overlay manager
service, there is some lead time before the affected process is updated.
Tests need to wait until this happens. Remove the calls to Thread#sleep;
instead, poll for a change to AssetManager#getApkPaths (which is faster
and more reliable).

Bug: 130257643
Test: atest OverlayDeviceTests
Change-Id: I9f7e6207a68431859bacac0462a2bab94b4efc2d