docs: Updated isAutofillSupported description.

The description for `AutofillManager.isAutofillSupported` doesn't make
clear that either the device or the user can make autofill unsupported.

Bug: 62604325
Test: Ran 'make ds-docs -j16' and staged content to
go/dac-stage/reference/android/view/autofill/AutofillManager.html#isAutofillSupported()

Change-Id: I298b9f535e23dc3cb54fabed36642523753c13a5