Accept MAINLINE_NETWORK_STACK permission on register/unregister network provider

Registering/Unregistering network provider needs NETWORK_FACTORY
which is signature only permission. But mainline modules can't
grant this permission because it's not signed with platform key.
Hence, these APIs should also accept MAINLINE_NETWORK_STACK
permission that allow mainline module to register/unregister
network provider.

Bug: 150733435
Test: atest FrameworksNetTests
Change-Id: Id22113aa0db01244817b4b008f5573c5e7b317f0
Merged-In: Id22113aa0db01244817b4b008f5573c5e7b317f0
(cherry picked from aosp/1248727)