Use an optimized findViewNodeByAutofillId(id) to get just one node.

Session was using findViewNodesByAutofillIds(ids) before, which not only was
not optimal, but error prone (for example, it could return a non-empty array
with an empty value).

Test: CtsAutoFillServiceTestCases pass
Fixes: 62532979

Change-Id: If984f1263cc3f2aac1d1e098687fe02d73c55211