Fix ANR in MultiRootDocumentsLoader

ANR might occure in MultiRootDocumentsLoader onReset because this
call on main thread. We call close task while onReset but task close
call will be blocked by runInternal then cause block on main thread.

Use Executor to let close task run in other thread to avoid ANR.

Fix: 126154768
Fix: 144074028
Test: manual
Test: atest DocumentsUIGoogleTests
Change-Id: I6ce6b9583d146b1e53784b4c7aec3e54f1808da0