Fix accidental implicit fallthroughs

-Wimplicit-fallthrough was printing warnings for these, they look
like real errors.  Fix them and re-enable the error.

Bug: 144040966
Test: treehugger
Merged-In: I818e18a995760f17b85b4c72ee577e531fa2a601
Change-Id: I818e18a995760f17b85b4c72ee577e531fa2a601
(cherry picked from commit 540824e658cc6df13c42faa548ef9ccb06aa0474)