Consecutive flyout animation

- fade out old message
- fade in new message
- vertical-center flyout w.r.t bubble

Bug: 170267642

Test: send single/group message
  => dot to flyout (and reverse) animation ok

Test: send consecutive messages from same/diff bubble
  => fade animation ok

Test: send multi-line message, then single line message
  => flyout updates vertical-center w.r.t bubble

Test: drag flyout to dot, tap flyout, repeat tests on right side
  => no regressions

Change-Id: I3e87c0ffebd27e1974b12ef4ad69bd1f627122ab