blue: Allow control of light notification timing

This implements control of light notification timing.

While we have a number of possibilities not implemented here,
the implementation has been limited to reduce code complexity.

Pattern duration is set to 8 seconds, resulting in each bit
in the 32-bit pattern being 250 msec.

Pattern repeat delay is set to 0 pattern lengths, so the pattern
always runs.

The actual blink timing is then controlled by altering the pattern
bitmask.  Blinking with a duty cycle >50% is not permitted
currently.  Handling blink period is a bit hacky - this should be
the off time, but in this implementation is the blink period
(total off+on time).

A Settings overlay is used to create duration values more friendly
to this implementation.

Change-Id: I5fe04677fd42edbaf1a444cb58d485fd150f2a83