Faster bugreports (2/n)

- Having a system property to disable parallel run function.
- Post 'DumpTraces' to the thread pool. 3% is
  improved compared with single thread run.

Bug: 136262402
Test: atest dumpstate_test
Test: atest dumpstate_smoke_test
Test: Manual enable and disable parallel run
Test: Manual trigger report using hardware key
Test: Manual trigger using bugreport shortcut
Change-Id: I2185cc3e2f429a150605d6268324c52d137db385