SnapshotManager::Return -> Return

Now that the class becomes more complicated, move it out
of SnapshotManager.

Also, make it independent of FiemapStatus. libsnapshot and
libfiemap has different return classes.

Test: libsnapshot_test
Change-Id: If340959ba60bcd51c36e2b48f4d11c149ead907a
9 files changed