AccessibilityManager#enableShortcutsForTargets permission adjustment

Changes how enableShortcutsForTargets checks permission, to allow the system process to call it successfully.

Also, moved the relevant flag code to a new dedicated flag.
Test: atest AccessibilityShortcutControllerTest && atest InvisibleToggleAccessibilityServiceTargetTest
Bug: 332006721
Flag: aconfig migrate_enable_shortcuts ENABLED

Change-Id: I13e45c7b667233412f38edb99e560d808b3d3601