Create central TelephonyListenerManager.

The TelephonyManager doesn't like have "too many" listeners
registered on int at any given time. It will actually throw
exceptions when this happens. To lighten the load from the SystemUI
side, TelephonyListenerManager now ensures that only one listener is
ever subscribed at any point.

SystemUI can now use this class instead, piggy-backing on the
possibly already subscribed listener to retrieve the events it cares
about.

Also, use Executors in CarrierTextController instead of Handlers.

Bug: 179775696
Test: atest SystemUITests
Change-Id: I626e80a91396161022e1fc6387598521f77bf4fc