Report boot statistics

Added the following metrics:
- tron_varz_boot_package_manager_init_start
- tron_varz_boot_package_manager_init_ready
- tron_varz_boot_system_server_init
- tron_varz_boot_system_server_ready

Test: manual - device boots, adb logcat -b events
Bug: 32780225
Change-Id: I0eeda831e2a13a7cbcef3308d5dad1bfa63919c1