libsnapshot: Re-expose InitiateMerge and ProcessUpdateState

These functions are used directly in update_engine now that
it is responsible for initiating the merge. update_engine has
its own message loop, so WaitForMerge / InitiateMergeAndWait
becomes useless and are deprecated.

Bug: 147696014
Test: compiles

Change-Id: Id8cb14f5f18b4893e2fc3d0d2b374a867a220e6f
Merged-In: Id8cb14f5f18b4893e2fc3d0d2b374a867a220e6f