ART: Reinstate secondary-image-patching exit

Do not attempt imageless running with a stale secondary image. Instead
prune the cache and exit, relying on init restarting the process.

The combined image checksum is not written down into the boot image
itself, only into apps. This should really be fixed.

Follow-up to commit a463b6a9.

Bug: 30832951
Bug: 69428309
Bug: 70213235
Bug: 70498154
Test: m test-art-host
Change-Id: I27ab3384ce4ffd93efb00a2e216cb839fe4357d5