Add getEmergencyRoleHolder SystemApi to RoleManager.

This API then enables TelephonyManager to expose the emergency
role holder to priviliged clients via getEmergencyAssistancePackageName.

Bug: 323157319
Test: atest RoleManagerTest
LOW_COVERAGE_REASON=Flagged test added but flag not enabled

Change-Id: I91089701248a32a720b488147405b2566cca2be9