Enable custom signal tones in LE mode.

Enable custom signal tones in LE mode.

This makes movement_play_signal synchronous when in LE mode, despite
using the underlying asynchronous API. It's a bit of a hack, but it
should work well enough for now.

This also moves the enabling/disabling of the buzzer into the
movement_play_signal function, so that watch faces no longer have to do
it.