Defer deleteKey in KeyStorage in Checkpointing mode

Don't delete keys in checkpointing mode. Instead wait until the
checkpoint has been committed.

Bug: 134631661
Test: Flash A with a working build. Flash B with a broken build. Test
      that the device rolls back to A without getting sent to recovery.

Change-Id: Ie5fc2d098355e2d095c53e9a95a6a8c7ab7ed051