Allow users to manually request Autofill.

- Added a requestAutofill(view,flags) method, that when passed with
  FLAG_MANUAL_AUTOFILL triggers a manual request.
- Added same method for virtual views
- Overloaded existing AutofillService request methods to take a flag.
- Added an AUTOFILL context menu option on TextViews.
- Added a canRequestAutofill() that is used to enable the context menu.

BUG: 35708229
Test: manual verification
Test: existing CtsAutoFillServiceTestCases pass
Test: android.autofillservice.cts.LoginActivityTest#testManualAutofill pass

Change-Id: I1a64d40da3373774451d178b1cabf20f11120e9d