altosui: Prevent voice altitude data from queueing up

When flight status changes rapidly, the queue of voice data can get
quite long. This change does two things -- first, it remembers when
the altitude reporting happens due to flight events and delays the
periodic reporting until a suitable time after that, second it ensures
that the voice data has all been delivered before generating a new
altitude report.

Signed-off-by: Keith Packard <keithp@keithp.com>