Add API to mark sanitized fields and implement it on TextView.

When sending a ViewNode to the auto-fill service, it's necessary to sanitize
it so it does not contain PII (Personally Identifiable Information).

A good rule of thumb is to mark as sanitized nodes whose value were statically
set from resources. Ideally, that logic should belong to View, but View does
not know how its contexts are set, so the decision must be made by the
subclasses, and this CL implements that logic on TextView (which is the main
auto-fill type - others will come soon...)

Bug: 33269702

Test: CtsAutoFillServiceTestCases pass
Test: manual verification

Change-Id: I4dcdd8419a35812bd2f15a63a5be85f8cd97bb4e