libsnapshot: add WaitForMerge

Add an API that does not initiate the merge, but only waits for it to
finish. It is different from ProcessUpdateState API in that it also
blocks when state == UNVERIFIED and booting from the new slot.
(ProcessUpdateState immediately returns in this case).

This is useful for android.os.UpdateEngine.CleanupSuccessfulUpdate().
Bug: 138808328
Test: libsnapshot_test

Change-Id: I7cc59fcaf69616e7ec7ebe6101991b5106845b65