Fix PatchScreen leak when moving between files

PatchScreen added a reference to itself to the shared copy of the
ListenableAccountDiffPreference for a change. Every file contained
by that PatchSet uses the same ListenableAccountDiffPreference,
so the PatchScreen and all of its DOM elements and widgets and even
the last cached PatchScript leaked across file navigation.

Bug: issue 1370
Change-Id: Iae588a34878f83d95bd1b03913d3a8523afdf58c
1 file changed