Create dialogs that handles changing magnification mode

The dialogs were created inside of
MagnificationModePreferenceController.
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: If120886091158ad22c746c0724225fb2a9ae5bd3