AppWidgetServiceImpl: Use enforceCallingOrSelfPermission for checking DUMP

Use enforceCallingOrSelfPermission() for throwing a security exception
rather than creating the security exception ourselves. This has the
advantage that the exception is in a well known format expected by
the CTS test android.security.cts.ServicePermissionsTest#testDumpProtected

Bug: 17165920
Change-Id: I7d3c2d2a0852a490fe1d553cd81d1efc79d2fec3