Get rid of kDebugAllowBundledKey

Instead, use PushFilePrepare to push test apexes into /system_ext/apex.

Also changed set up logic for apex activation tests, because it relied
on the fact that test apexes were not pre-installed.

Test: atest ApexTestCases
Bug: 154908224
Change-Id: I56b0d7e3e59dcf246edd5e1fed0dd0122bd3069e
6 files changed