[A11y][Keyboard shortcuts] Update Magnification dialog strings

* Update the Magnification dialog strings to accurately reflect the
  behavior of the dialog.
* Refactor the Magnification dialog logic out from the remaining AT
  since Magnification's dialog behaves notably differently from the
  rest.

Bug: 436622353
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Test: AccessibilityShortcutsRepositoryImplTest, manually tested
Change-Id: I1719cf0c7c59e32c865b1ee1b710a77be983172c