Adds @hide ImsService APIs

Adds @hide ImsService API implementations to be used for the new
dynamic ImsResolver.

1) ImsService - The main class that all vendor ImsServices will implement.
ImsServices that implement this method must return their implementations
of MMTelFeature when onCreateMMTelFeature is called. The base ImsService
class also relays all method calls through itself as a proxy. So, when
Telephony calls a method, the ImsService figures out which MMTelFeature
should be called (by slot) and then calls that feature's method
implementation.

2) MMTelFeature/RcsFeature - Implements the I*Feature interfaces, which
are used on both sides of the interface. The vendor implemented ImsService
must implement all methods provided in the I*Feature interface in their
implementation of *Feature that they return to the ImsService.

3) ImsServiceProxy[Compat] - The Proxy interface in telephony that will be
called in ImsManager. When a method in this class is called, it will call
the respective AIDL function: Telephony -> IImsServiceController AIDL ->
vendor ImsService -> vendor ImsFeature implementation.
ImsServiceProxyCompat is there to provide backwards compatibility with
older ImsServices that do not use the new ImsService implementations.
It implements all of the methods that are defined in the new I*Feature
interfaces and translates them to the old ImsService AIDL calls.

Test: Adds Unit Tests (see frameworks/opt/telephony)
Change-Id: Id3466c178384158c788ab1d708ab108bb95866fc