Fixed basic autofill services broken on previous CLs.

HeuristicsService: don't generate generic hint on non-autofillable fields,
  otherwise it will never trigger Save because it would be expecting changes
  on fields like buttons.
BasicService: scan children as well.

Bug: 114236837
Test: manual verification

Change-Id: I87c335737386f040fec4b036203756cbcaec752c