Move perm control svc impl methods on async task

Now that PermissionControllerService calls everything on the binder
thread, the impl has to move the calls onto a thread pool.

Run backup on single thread so that the delayed backup state won't get
overridden.

Bug: 131314749
Test: atest --test-mapping packages/apps/PermissionController/src/com/android/packageinstaller/permission/service:presubmit
Change-Id: I2c3a6cd01cd9f599a82a6ffd56632aad701395ef