[A11y][Keyboard shortcuts] Update and flag Magnification dialog strings

* The dialog strings for the Magnification keyboard shortcut will be
  different depending on whether or not the keyboard shortcut will be
  enabled, which is flagged behavior. This CL updates the strings to be
  flagged accordingly.
* Update strings to latest copy from UXW in alignment with other AT.
* Minor renaming of methods for clarity.

Bug: 425722546
Flag: com.android.hardware.input.enable_magnify_magnification_key_gesture_dialog
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Test: AccessibilityShortcutsRepositoryImplTest, manually
Change-Id: If175953bfb4bad841fa2c142302d15976447a32e