Suppress -Wimplicit-fallthrough in aapt code

AaptAsssets intentinally falls through in a switch statement.
This change adds [[fallthrough]]; to suppress this implicit fallthrough.

frameworks/base/tools/aapt/AaptAssets.cpp:305:17: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
                case 5:
                ^
frameworks/base/tools/aapt/AaptAssets.cpp:305:17: note: insert '[[fallthrough]];' to silence this warning
                case 5:
                ^
                [[fallthrough]];
frameworks/base/tools/aapt/AaptAssets.cpp:305:17: note: insert 'break;' to avoid fall-through
                case 5:
                ^
                break;

Test: make -j aapt
Change-Id: Ia9d2ca3b59eb804e20c6295791f6a19f0f5f63ab