Don't rely on fallthrough in incident_section_gen

Silences the following warning during build:

frameworks/base/tools/incident_section_gen/main.cpp:265:13:
    warning: unannotated fall-through between switch labels
[-Wimplicit-fallthrough]
            default:
            ^
frameworks/base/tools/incident_section_gen/main.cpp:265:13:
    note: insert 'GOOGLE_FALLTHROUGH_INTENDED;' to silence this warning
            default:
            ^
            GOOGLE_FALLTHROUGH_INTENDED;

frameworks/base/tools/incident_section_gen/main.cpp:265:13:
    note: insert 'break;' to avoid fall-through
            default:
            ^
            break;

Test: make -j50 2>&1 | grep incident_section
Change-Id: Ie59510025e59e96f0763726794ff19dae595c672