[DO NOT MERGE] Guard against launching permission backup/restore twice per request

It looks like there is some case where AbstractRemoteService would #run the
same PendingRequest the second time which violates AsyncTask assumption.

Fixes: 135986503
Test: atest android.permission.cts.PermissionControllerTest
Change-Id: I2d072714ee0fad3c365ff7eb191d690056c0931a