Allow implicit-fallthrough warnings locally.

It will be a global error by default.
Cherry pick lost changes from AOSP, for Android.mk and main.cpp.

Test: make checkbuild
Bug: 112564944
Change-Id: I6f1024fa5e31302af27fa9d5bac195e511f4ba20
Exempt-From-Owner-Approval: do not block on new warnings
3 files changed