Support featureFlag attribute in <assign-permission> tag

My team is adding a new runtime permission that we want to grant to special system UIDs in platform.xml. We will need to guard these exemptions behind a flag, because the permission isn't available yet. I didn't add a flag because this is straightforward and could have been added along with ag/30189176. I added unit tests for this case and the split-permission case as well.

Bug: 439687717
Flag: EXEMPT bugfix
Test: atest FrameworksServicesTests:com.android.server.systemconfig.SystemConfigTest
Change-Id: If362e36169633d9ed2e14099d1c743f3c4bcd7e8