Create DialogFragment for handling changing cursor following mode

The dialog was created inside of
MagnificationCursorFollowingModePreferenceController.

Moving them outside of the controller makes it:
- Easier to read
- PreferenceController is only responsible for displaying the preference
- Easier to test dialogs

Bug: 406052931
Test: atest
Test: manually by changing the code locally to show the dialog
Flag: EXEMPT no real usage yet
Change-Id: I2b5c6449183d82f9504fa05632b740950744a1ee