Fix doc for Telephony Manager api and make isTtySupported public.

TelephonyManager.isTtyModeSupported is public and we are deprecating that
to use TelecomManager.isTtySupported which was system api, so making it
public.

Test: Build
Bug: 62346128
Change-Id: Idfa146b03a3c3fdb20c563c4ea0dddf2c3399da6