Use nanos and boot time in SnoopLoggerTracing.

Perfetto uses CLOCK_BOOTTIME and nonoseconds by default for everything.
This change replaces all time usage with perfetto::base::GetBootTimeNs
to ensure we're doing what Perfetto expects.

This change also fixes the trace packet timestamp. Bundles can be emit
some time later, so using the current time is incorrect. Instead, use
the bundle start time.

Bug: 365778358
Test: m sync
Test: build, flash, and record trace
Change-Id: I84313c20de2a562ce2f484d1d81341c07639f771