Enable -Wimplicit-fallthrough

We're trying to enable -Wimplicit-fallthrough throughout Android. This
warning requires an explicit annotation every time we fall through
intentionally.

Thank you for explicitly commenting your fallthroughs :)

Bug: 112564944
Test: Builds

Change-Id: Iae8081a9776562330dd4b94d1091893086c984ad
2 files changed