Save the user preferred shortcut on Edit A11yShortcut screen

We saved the user preferred shortcut in the a11y feature's detail page.
Since now the Edit A11yShortcut screen is a standalone screen, we might
want to keep the existing behavior to update the user preferred shortcut
types when the edit screen is shown or when the shortcut settings are
updated.

Bug: 300302098
Test: atest
Test: manual
    - Go to an a11y feature page and open the Edit shortcut screen
    - Remove all selected shortcut option
    - Select volume key shortcut option, and deselect volume key
      shortcut option
    - Go to home page and close the Settings app
    - Go to the same a11y feature page and click on the shortcut toggle
    - Expect the volume key shortcut is turned on
Flag: ACONFIG
com.android.settings.accessibility.edit_shortcuts_in_full_screen
TRUNKFOOD

Change-Id: I2c92ec56d6e45317406f3a4b8ef8f076c398df7f