Allow specifying "platform" as cert for default permissions

Platform signed apps don't have to write signature to the
default-permisson xml file because system already knows it.

Bug: 349926603
Flag: NONE simple upstream from OEM
Test: Manual
(cherry picked from https://partner-android-review.googlesource.com/q/commit:34753ead096e5175ece11d6606e1642e9494a50d)
Merged-In: I79d0729bfcf756949bf7e71e3ef542bedf4545f3
Change-Id: I79d0729bfcf756949bf7e71e3ef542bedf4545f3