Implement Job.getWarningNotification()

Put a default implementation of getWarningNotification() in the Job base
class. This removes the duplicated implementations of this method from
DeleteJob and TestJob. This will also simplify the implementation of the
upcoming UnpackJob class.

Bug: 406328303
Flag: EXEMPT refactoring without any functional change
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services
Change-Id: I74ecc6e9d8247ad6f32f139a371ef9dbb7ed4229