Clarify PhoneStateListener documentation

onXStateChanged() methods are called when the X state changes, but also
once on initial registration. This is called out in the documentation
for TelephonyManager.listen(), but we should clarify it here too.

Fixes: 124204168
Test: no change to behavior
Change-Id: Ie0fe8c90ff5610597f810d6ba8425138b8a286ee