Preserve text field's caret position across focus changes #86752

Currently we set the selection and the composing range of a text field to empty when it loses focus (#10911):

Steps to Reproduce

But on Android if you:

1. go to a login screen
2. type something in the password field, and move the caret to the middle of the string,
3. select the username field (the caret should disappear), and use the "next" button on the virtual keyboard to focus the password field again
4. As the password regains focus the caret re-appears, and it should be where it was.

GTK seems to have the same behavior, except it doesn't collapse the selection while android does (maybe it's the IME?). When you press tab to refocus an input field that previously has a non-collapsed selection, the selection will re-appear.

Dropping the selection also notifies the text field's TextEditingControllers listeners (#79982). Not commonly expected.