libandroidfw: add FALLTHROUGH_INTENDED to silence warnings

Add FALLTHROUGH_INTENDED to silence compiler warnings such as the
following:

-------- 8< --------
[ 42% 27/63] //frameworks/base/libs/androidfw:libandroidfw clang++
Locale.cpp
frameworks/base/libs/androidfw/Locale.cpp:166:7: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
      case 5:
      ^
frameworks/base/libs/androidfw/Locale.cpp:166:7: note: insert 'FALLTHROUGH_INTENDED;' to silence this warning
      case 5:
      ^
      FALLTHROUGH_INTENDED;
frameworks/base/libs/androidfw/Locale.cpp:166:7: note: insert 'break;' to avoid fall-through
      case 5:
      ^
      break;
1 warning generated.
-------- >8 --------

Test: 'make libandroidfw' builds with no compiler warnings
Change-Id: I6bc67f2c072f88c9f1a92b7378a6f1a460db70b4