Improve cancellation tests related to pending jobs

This catches the bugs fixed in ag/32964004 by having the tests cancel
jobs via the service rather than directly on the job, and also adding a
test to test cancelling jobs with jobs queued up.

Bug: 400352714
Test: atest -c 'DocumentsUIGoogleTests:com.android.documentsui.services.FileOperationServiceTest'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I73d3b0245ec257feaf1f4d2ff133e67e0215d0a3