Save RTT chat history during the call.

Thus putting call into background won't lose the history.
This is only stored in memory by making RttChatMessage Parcelable and saved
along with fragment's life cycle.

Bug: 67596257
Test: manual
PiperOrigin-RevId: 188500104
Change-Id: I11e8e55f0475defd9c3b9a8cc10db4186392ddd8