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

Change-Id: I4d964adc826e62c29ab73a098474a116f9818366