[A11y][Keyboard shortcuts] Update Voice Access dialog strings

* Update the Voice Access dialog strings to accurately reflect the
  behavior of the dialog.
* Refactor the Voice Access dialog logic.

Bug: 438518742
Flag: com.android.hardware.input.enable_voice_access_key_gestures
Test: AccessibilityShortcutsRepositoryImplTest, manually tested
Change-Id: I7b8c30cc122187189bc0d422b375a7e742610afe