Use clock monotonic for AppStartInfo timestamps

Currently timestamps are a mix, need to pick one

Why clock montonic:
- This is what's documented, used of elapsed time is really a bug.
- This is what's used across most of logging, enabling us to be consistent and use available timestmaps (as we already are) more easily.

Also stop using and remove tracker single timestamp methods in favor of using broader one with key specified.

Test: check timestamps via shell command
Bug: 338637327
Flag: EXEMPT - day-to-day bugfix
Change-Id: Id3f242cf717c967d94e813ef3ee63ede2a374068