Implement read/write text stream to RttCall.

This change also:

1. Add simulator support of RTT request during call (always accept at this moment, will add random accept/decline in the future)
2. Fix bugs of putting RTT call in background and back to call

Bug: 67596257
Test: Simulator
PiperOrigin-RevId: 185920527
Change-Id: I51016fa6cf1ccc8a5a21335f9dacf286ae393706
12 files changed