Faster bugreports (5/n)

Post 'DumpCheckins' and 'DumpAppInfos' to the thread pool.
10% 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: I107b3de9d49309fbfa1256203bfe705ca6c7608d