systrace: Add more atrace sections.

Add several atrace sections, most important to determine if iorap
is prefetching or perfetto-tracing.

Also adds atrace sections for the background job maintenance code.

Use the 'am' tag for activity-driven events,
and the 'pm' tag for the background job code.

Test: systrace.py -t 10 pm am
Change-Id: I1ed695ddee2990db9df8a2736808f4d9fc4e4fb4
Bug: 150644675
8 files changed