Add integration tests for BackgroundDexOptService

Add three tests:
1. Under normal conditions, check that dexopt upgrades test app to
$(getprop pm.dexopt.bg-dexopt).
2. Under low storage conditions and package is unused, check
that dexopt downgrades test app to $(getprop pm.dexopt.inactive).
3. Under low storage conditions and package is recently used, check
that dexopt upgrades test app to $(getprop pm.dexopt.bg-dexopt).

Test: atest -v BackgroundDexOptServiceIntegrationTests
BUG: 64807719

Change-Id: Iaa50d5120ea0255b38226bda0452e7e47f1ff5d0