ueventd: Run external handler as non-root group

The external firmware handler always has root group privileges because
it is forked/executed without setgid() by ueventd which has root
privileges. This patch calls setgid() with group ID specified in
ueventd.rc before execv().

Test: atest CtsInitTestCases
Signed-off-by: Suchang Woo <suchang.woo@samsung.com>
Change-Id: Id1430e783b0e409d55ac80fe213e81ba099729e2