Background mode for busy_threads
Background mode for busy_threads

Add --background flag to run busy_threads in background. This makes it
easier to test CPU tracing on battery (with USB cable disconnected).

The flag is handled in the same way as in perfetto_cmd.

Test: ./busy_threads --background
Bug: 159329054
Change-Id: I2deb2aab3fad0edebd032a0222b21a51051c49e2
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto