Rename and publicize getMaxPhoneCount; and deprecate getPhoneCount.

To have better names reflecting what getPhoneCount / getMaxPhoneCount
actually represent, we replace them with getActiveModemCount and
getSupportedModemCount. As getPhoneCount is public API, we mark
it as @Deprecated.

Bug: 141388730
Test: build
Change-Id: If086c50ae803b384691f709057bceeac2fc1d9d5