[Autofill PCC] Fix authentication logic for prefer PCC

Fix the authentication filling logic when prefer PCC is enabled.
Previous internal logic could create multiple datasets after authenticaiton.
This problem was visible if the PCC didn't detect any of the filling fields.
The bug is fixed by merging the datasets with the detected fields, as to
create a single dataset.

Bug: 293313411
Fixes: 291194194
Test: atest CtsAutoFillServiceTestCases
Change-Id: I5dd50c6639a96a046213e00fddf0b15c8bb3d5e3