Fix NPE when PermissionEntry.getGids() returns null.

PermissionEntry.gids may return null when parsing legacy config trying
to work around an issue about upgrading from L platfrm. We can just
ignore such entries now.

Bug: 325137277
Bug: 322197421
Test: system boots with flag enabled
Change-Id: I71a160d53b1a1b429019bb1973a892f78ccb0a17
Merged-In: I71a160d53b1a1b429019bb1973a892f78ccb0a17
(cherry picked from commit b0b149c4)