Allow privileged apps to accept and end VOIP calls.

This change updates relevant docs to describe that TelecomManager APIs allow privileged app invocations to accept and end VOIP calls. This CL also adds support for checking if an app is the sytem UI app which is considered privileged. This functionality was requested by the accessibility team in order to support their talkback double tap gesture feature.

Bug: 353579043
Flag: com.android.server.telecom.flags.allow_system_apps_resolve_voip_calls
Test: atest TelecomServiceImplTest
Change-Id: Ia0a8dbd7bcdd88b5a8cc4b0f908f935d77f6c734