Replace `time.time` with `time.monotonic`

We should replace all usage of `time.time()` with `time.monotonic()`.

`time.time()` is influenced by the system clock. This means if the system clock is changed during fit, then AutoGluon could incorrectly run for longer or shorter times than intended.

`time.monotonic()` does not care about the system clock, and should be more trustworthy.

https://docs.python.org/3/library/time.html#time.time
https://docs.python.org/3/library/time.html#time.monotonic

Kudos to @eddiebergman for making us aware of this.

Search the repository for every use of time.time() and inspect the timing paths around fit. Confirm each elapsed-time calculation uses a clock unaffected by system-clock changes, then run the tests covering the affected components and verify fitting durations remain correct.

Written by the indexing model from the issue text.