Clean up update state when snapshots are interrupted or cancelled.

This patch addresses two edge cases with interrupted updates:
 (1) If the device reverts to the old slot *before* merging, snapshots
     must be removed.
 (2) If during a merge, a snapshot is detected to be invalid (due to
     flashing), the snapshot must be removed.

To encapsulate this logic, WaitForMerge has been renamed to
ProcessUpdateState. It is still intended to be called after /data is
mounted, or after a merge is initiated.

Bug: 139154795
Test: libsnapshot_test gtest
Change-Id: I37a25ca722f30ae9548894dcfbd70cb64330e416