Move Track::signal to TrackBase

The implementation of Track::signal() is not tied to playback tracks in
any way. Pull it up into TrackBase, so it can be re-used by Mmap tracks.

Test: compiles
Bug: 376481063
Flag: EXEMPT mechanical refactor
Change-Id: I768b4a584942671ba005697d1a3cc1e6d121d86d
4 files changed