Use explicit executor when factory reset a device.

By default, AsyncTask uses a SerialExecutor which can be blocked by
other AsyncTask indefinitely. This sometimes can cause the factory reset
operation to be blocked for a long time. This change explicitly changes
the executor to THREAD_POOL_EXECUTOR so factory reset operation won't be
blocked.

Flag: EXEMPTED - small bug fix
Bug: 367798236
Fix: 367798236
Test: manual test factory reset flow
Change-Id: I99f5046cfc6d8a6c9a7be7bda950103b859f7489