Add video calling setting to TelephonyManager.

We want to add a user setting to enable/disable video calling. This
is a telephony-level setting which the user sets using Telephony's
Call Settings, and applies universally to video calls.

But this setting needs to be accessible in places such as Dialer,
so these changes are to support storing the setting in
TelephonyManager (to be implemented in PhoneInterfaceManager) and
providing access to the setting for applications like Dialer
through TelephonyManager.

+ Add function definitions to ITelephony.
+ Add function wrapper implementations in TelephonyManager.

Bug: 16014284
Change-Id: I1dce8bf5ae4f0ee115698ce9627ba8fc424dd855