Add getEmergencyAssistancePackageName API to TelephonyManager.

Since the emergency role is basically the app that handles
ACTION_EMERGENCY_ASSISTANCE, it was proposed to add this API
alongside isEmergencyAssistanceEnabled(). This provides privileged
clients (e.g. GmsCore) with a robust way to query the emergency
role holder.

This API calls a new getEmergencyRoleHolder() API in RoleManager.
Both of these APIs are guarded by a common flag in the permissions
namespace.

Bug: 323157319
Test: atest RoleManagerTest
Change-Id: I4302ca53f6a0187f225ad085a7d43cd20a935b3e