Add a documentation in EmergencyNumber#getNumber

If the number starts with a carrier prefix, the
 carrier prefix is configured in
 {@link CarrierConfigManager#KEY_EMERGENCY_NUMBER_PREFIX_STRING_ARRAY}.

This is important because it provides a solid reference for Apps to
strip the prefix, which they may not prefer to display.

Bug: 137225076
Test: Treehugger
Change-Id: I1f6bd74f1d6eb86fe3326246e0ddb08b9cf7ac11