Exempt platform-cert signed apps from hidden API checks.

This means that APKs signed with the platform cert are allowed to use
hidden APIs, even if they are not on the package whitelist, and if they are
not in the system image. It will also allow a number of packages to be
removed from the package whitelist.

Also remove all platform cert signed apps from the package whitelist, as
there is no longer any need for them to be in there.

Bug: 64382372
Test: device boots
Change-Id: Id805419918de51f946c1f592581bab36ae79de83