nfc: enable -Wimplicit-fallthrough

We're trying to enable -Wimplicit-fallthrough globally, which requires
FALLTHROUGH_INTENDED when a fallthrough is intended.

Thank you all for explicitly commenting your fallthroughs :)

Bug: 112564944
Test: Builds
Change-Id: Ib52428ce8165762ec4750a56a848d2db8920cc48
5 files changed