Make BUGREPORT_FLAG_CAPTURE_MULTI_DISPLAY_SCREENSHOT flag to System API

Change BUGREPORT_FLAG_CAPTURE_MULTI_DISPLAY_SCREENSHOT to system API by
removing hide tag

Test: Local build
Bug: 364767133
Flag: android.os.bugreport_multi_display_screenshot_enabled
Change-Id: Ibbab058009423d7bbfdc7144b4c15e2564fc20dd