Allow AutofillService can identify if fill dialog presentation is needed.

If the fill dialog is available for the field, the FillRequest flags
will contain FLAG_ACIVITY_START. The field is hidden originally but
it needs to be get from AutofillService to avoid creating unnessary
fill dialog presentation. Rename the flag because the it's not clear
to combine the relationship with fill dialog.

Bug: 223472039
Test: manual. Build and boot pass.

Change-Id: I52c367e75100f6af2ab782f58755bc5727b1dd72