Start using new reserved disk GID.

We recently created a new GID that can be granted to critical system
processes, so that the system is usable enough for the user to free
up disk space used by abusive apps.

Define a permission for the GID so we can grant it to system apps,
and add the GID to core apps needed for system stability.  (The list
was mostly derived from filling a disk and seeing what caused the
device to fall over.)

Test: builds, boots
Bug: 62024591
Change-Id: Icdf471ed3bed4eeb8c01f1d39f0b40c1ea098396