Introduce @hide TextView#setTextOperationUser()

This CL introduces a unified way for framework developers to specify
whose components should be interacting with the given TextView.  An
important use case is the direct-reply UI notification hosted in
System UI, which always runs as user 0 no matter who is the current
user.

For instance, to let the given EditText interact with user 10's input
methods and spell checkers, you can call setTextOperationUser() as
follows.

  editText.setTextOperationUser(UserHandle.of(10));

In this way we can easily propergate the same user ID to other
components such as autofill and text classifer as necessary in the
future.

No one calls TextView#setTextOperationUser() yet hence there should be
no user-visible behavior change.

Bug: 120744418
Bug: 123043618
Test: spell checker still works
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I6d11e4d6a84570bc2991a8552349e8b216b0d139