Log length of grace period used to start Activity

If the Activity start is only allowed by grace period (neither creator
nor sender allow with another BAL code) log the length of the grace
period used. With the collected data (from droidfood) we can determine
what a reasonable cutoff would be.
Also adds a flag for reducing the grace period as soon as we have collected sufficient data.

Test: atest BackgroundActivityLaunchTest
Flag: EXEMPT log only update
Bug: 362575865
Change-Id: I2b18caf87b63c32c763060b72062c8ba44637063