IMS: RTT feature changes

Implement RTT feature changes

IMS: Allow sending RTT message in Upon Request mode
Scenario -
-Device is in Upon Request mode
-Device receives RTT call and call is now active
-User tries to send RTT message
-Allow sending the message in this mode
-Remove the full mode check

IMS: Add null check for RttTextStream

RttTextStream can be null if RTT operations are not
performed from UI. So add a null check to prevent
NPE when the operations are performed through adb
commands.

IMS: Answer the call as RTT only if call is offered as RTT

Waiting call is answered as RTT call, even if the call is offered
like a regular waiting call if RTT UI option is on.
To fix this, check if the call profile is having RTT mode before
answering it as RTT call.

IMS: Handle RTT Downgrade request and response

Handle RTT Downgrade request and response.

Change-Id: Ied8086f04ec76ad2cd3712ff902a64d747deae1f
CRs-Fixed: 2178403