fix: power brownout and dirty write flush on restart or shutdown

[NickDunklee]

This is probably another "needs soaking" fix as it touches power.

Backstory on this one:

I noticed the sensor firmware build was aggressively sending
"Battery is low" messages constantly when a RAK19007 was below
50%. (These messages only show up in third party clients to all
node admins, as the stock MeshCore mobile client doesn't let
one see messages from a sensor node. Seems another power draw
sending that message, but not part of this PR.)

Then people on the local mesh have been on and off talking
about certain nodes randomly losing their contact lists on
some node types, and others were talking about Heltec V4
brownouts. I also observed Heltec v4 die prematurely around
50% and I started thinking they were all related.

Started digging into the code and found a few potential leads:

• MeshCore does a "lazy" write on dirty_contacts_expiry
  in a 5 second window.
• The shutdown/restart path do not clean this up
• Low battery check is a poll every 8 seconds with
  no awareness of other things going on in the node

On the power piece:

Heltec V4 and other higher-powered nodes can hit the battery
harder when transmitting, below 50%, lithium batteries sag
more dramatically than they do at higher charge states.

If the power check happens at the same time as transmit,
the shutdown code gets called prematurely and shuts down
the node.

On the file write piece:

If the shutdown or restart paths are called, the code just calls
shutdown() or reboot() without checking and calling
saveContacts(). There do not appear to be any other file writes
that act this way.

The Fix

The change is kept using AUTO_SHUTDOWN_MILLIVOLTS so it respects
previous power threshold decisions across all node types.

With this change, all restart or shutdown paths will make
sure to call saveContacts() before shutting down to stop
the list from becoming corrupted.

It also suspends reading battery level for 250ms during transmit
(adjustable) so a power sag doesn't trigger an early shutdown.

On Heltec V4 at least, the MeshCore software power threshold is
much higher than the board's internal brownout/shutdown threshold.

Tested on

• Heltec v4
• RAK 19007
• Heltec T096
• RAK 19003

On the Heltec v4, I can now pass 50% and get down to 36% before it shuts
down. Although the voltage at 36% should probably actually say 5%
based on some voltage curve sites like this one.

That is probably an idea for future mobile app improvements, the MCU
temp and battery voltage could be used to calculate the battery percentage
in the app itself and it would likely seem a bit more "accurate"
on all board types without having to add math in the node code.