Update RTT transcript screen.

1. default to show first message
2. fix bug that first timestamp is not full format when it's grouped with previous messages

Bug: 70177426
Test: RttTranscriptAdapterTest
PiperOrigin-RevId: 193542992
Change-Id: Ic7a00f1fad8a9305b6fc1111783d589cc7956d58
2 files changed