vold: Grant root group

When using make_f2fs to format a filesystem with quota enabled, make_f2fs
will write quota record with group id equtal to make_f2fs's execution
group id.
Vold shall run as root group so that make_f2fs forked by vold will enherit
vold's root group. Make_f2fs can then write quota record with group id for
root group and therefore kernel quota driver can find quota record for
root group.

Bug: 111775499
Change-Id: I0c1fef760597d1695586d24365200b25e2cf07e6
Signed-off-by: Light Hsieh <light.hsieh@mediatek.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>