Add tracepoint to track Launcher cold startup latency

We want this for the post-boot performance analysis as an
rough but easy to understand metric for time spent in post-boot.

Bug: 299342732
Test: Reboot phone, before lock screen start a trace
Test: Run the following SQL against the trace: select * from slice where name="LauncherColdStartup"
Change-Id: I8956368338b13cb059ba7086e74b4c7f25849be2