Expose and update the call forward/waiting APIs

Expose the call forwarding and call waiting APIs, update their
documentation, and convert them to async APIs due to their dependence on
network communication.

Fixes: 168655869
Test: atest android.telephony.cts.TelephonyManagerTest
Change-Id: Ic20bc99314c8538a3f75f6af9c37bc4d30781825
Merged-In: Ic20bc99314c8538a3f75f6af9c37bc4d30781825