Expose MmTelCapabilities#isCapable() as Public API

Expose the MmTelCapabilities#isCapable() API to complete
a chain of APIs exposed for debuggability in Android RVC.

Without this API, there is no (public) way for a caller that
receives a capability callback to determine whether a particular
MMTel feature is enabled for a particular IMS Connection.

Bug: 156107100
Test: make update-api && make offline-sdk-docs && make
Test: cts - ImsServiceTest#testCapabilityStatusCallback()
Change-Id: I0a0d72897112460370e6981ae33cef4da6e3ce4b