Refactor AnalyzerConfig to use Timestamps instead of microseconds.

Add optional offset-to-UTC parameter to output. This allows aligning
the x-axis in the generated charts to other UTC-based logs.

Bug: b/215140373
Change-Id: I65bcd295718acbb8c94e363907c1abc458067bfd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250203
Reviewed-by: Kristoffer Erlandsson <kerl@google.com>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35992}

rtc_tools/rtc_event_log_visualizer/proto/chart.proto

@@ -33,4 +33,8 @@ message Chart {
 
 message ChartCollection {
   repeated Chart charts = 1;
+  // `calltime_to_utc_ms` is the UTC time (in ms) for the x-axis in the charts.
+  // In other words, time t ms in the charts corresponds to
+  // t+calltime_to_utc_ms ms in UTC time.
+  int64 calltime_to_utc_ms = 2;
 }