Implement the missing functions `xTimerGetExpiryTime()` and `xTimerIsActive()`
for the move of `TimerController` to use FreeRTOS timers.
In the process also fix a slight inaccuracy in the ms to ticks
conversion (no conversion was done as 1000 was close enough to 1024 for
a first working implementation).
Fixes: https://github.com/InfiniTimeOrg/InfiniSim/issues/17