SnapshotManager::WaitForMerge gives more info

It now returns Ok() if successful, NeedsReboot() if merge
it should be checked again after reboot, and Error() for
other errors.

This wraps UpdateState to help clients interpret the UpdateState value.

Also separate SnapshotManager::Return from FiemapStatus since they are
for different libraries and have (potentially) different set of error codes.

Test: libsnapshot_test
Bug: 138808328
Change-Id: I8c95417c2b0b7b2a362beb12585f861453a79278