Allow TTY mode for GSM Phones

TTY mode should not be restricted to CDMA phones as some GSM carriers
support it.
TTY support is enabled by overlaying the tty_enabled boolean property
in packages/apps/Phones/res/values/config.xml

Also corrected wrong comments on TTY methods.

Change-Id: I48dbc2be51c3dcdaedc1838b85134edc7012be3c