Turn off automatic heap dump notifications in SysUI

You can reenable them on any device with

  adb shell setprop debug.enable_sysui_heap_limit 1

The user-initiated dumps are still available via the memory
tile, which will remain on debuggable builds.

Fixes: 159212013
Test: n/a
Change-Id: If261427a217911b8dd6d82a51106055a4b6e5d02