Additional boot metrics

Extracted duration measurement functionality into BootTimingTraceLog.
It is now shared between system_server and zygote.

Log the following metrics to tron:
- boot_zygote_init - Time in milliseconds to boot into Zygote init stage.
- boot_android_init - Time in milliseconds to boot into Android init stage.

Test: manual - device boots, metrics are logged to system/event log
Bug: 32780225
Bug: 31115337
Change-Id: I600ac7fc83d35fa226ac92c37cc4b19192b25f59