Prune image cache if the boot marker is still in place.
We place a marker in each isa cache dir and have the framework
delete it for us.
This seems like a convenient place to do it given all the other
dalvik-cache related code that hooks in here, and also because
this is the first point where we try to access the cache.
TODO: Add a timeout to the (higher level) zygote code to correctly
kill the surviving zygote if one of them never comes up.
bug:
18280671
(cherry picked from commit
c38276177aeeda4326a54f1121790c154df04300)
(partial cherry-pick of
9433ec60b325b708b9fa87e699ab4a6565741494)
Change-Id: I5dbea21b4bf9ca4106a13225946d3f35f8060a43