[PCC]: Fixes for PCC

Create session only once. Previously, multiple sessions could be
created, and can result in multiple requsets to PCC.
Add local logs for debugging.
Fix to show datasets from PCC. Seems like it stopped working from a
previous change. Upcoming PccFieldClassificationTest should prevent
such regressions.

Test: atest CtsAutoFillServiceTestCases
Also atest android.autofillservice.cts.servicebehavior.PccFieldClassificationTest --iterations=5
Bug: 279608578

Change-Id: Ia026adc6e45ea3d7c15c560fac59093f202ccc4e