Fix gid not being set in ueventd when device nodes already exist.

The chown() call will fix the uid for pre-existing nodes, but not the
gid. This fix ensures the correct gid is set if needed.

Bug: 187738549
Test: manual test modifying ueventd.rc
Change-Id: I0fadd745a7c57a089fed9afc2572ace597a05396