Migrate RTT chat history to use proto buffer.

This change also moved chat history to be saved in DialerCall, thus it will be persist when switching calls.

Bug: 67596257
Test: RttChatMessageTest
PiperOrigin-RevId: 191626079
Change-Id: I65e9c6ac7aa41beb45dfe561b0a48506ea938e6f
7 files changed