A new setting key to hold targets of the a11y button

Roll back the changes of ACCESSIBILITY_BUTTON_TARGET_COMPONENT to
support previous behavior of the a11y button. Also, create another
setting key to hold shortcut targets of the a11y button which is
assigned by the user from the settings app.

Bug: 152263232
Test: atest SettingsProviderTest
Change-Id: I7a70e8c8495cefe6632efddbb322a323aa1168c7
Merged-in: I7a70e8c8495cefe6632efddbb322a323aa1168c7