[system][bt] address -Wimplicit-fallthrough

The compiler upgrade in b/139945549 can now check C code for
-Wimplicit-fallthrough. In b/143713277, were moving the cflags for
projects that disable this warning to cppflags. For this subsystem, just
address the warnings and remove disabling it.

These cases should be carefully reviewed; modifications that weren't
additions of fallthrough attributes were likely existing bugs.

Bug: 139945549
Bug: 143713277
Test: mm
Change-Id: Icafeed1d18991e83f42e5efd13e8ab8d15068d79
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>