Better handling of system time changes

[wellenvogel]

Currently AvNav on Linux is at least prepared for system time changes (including backwards). But the reaction is rather "hard" - all internal GPS data is deleted if a time shift backward is detected.
On Android there is currently no real handling.
But the systems provide the ability to rely on some steady clock sources (time.monotonic for python and System.uptimeMillis() or System.elapsedRealTime(9 on android).
So AvNav should utilize those time sources. This can avoid the need for deleting internal data.
Also the browser part should be considered (maybe using performance.now())

[wellenvogel]

20240520