runtime: Properly unload partially loaded image spaces

When one of the (non-app) image space successfully loads,
it sets up runtime callee-save methods.

If it is later unloaded, callee-save methods are now pointing to memory
that is no longer valid (viewed as all 0s in the debugger).

Runtime::Init skips creating its own runtime methods if it already sees
that the runtime methods were set to non-null, thus dangling runtime
methods.

This crash would nominally manifest itself in unwinding the first time, or as a DCHECK
failure in the interpreter bridge invocation during aborting if debugging was enabled.

To get into this state:
* Fill up the /data partition (but perhaps leave a little bit of room
for one image, but not all images)
* Reboot the device or run zygote manually.

Test: adb shell dd if=/dev/zero of=/data/local/tmp/tempFiller.deleteMe bs=1024 count=50g ; adb reboot
Bug: 36033084
Change-Id: I728c1058b003fcf5e98dc2746d53e44b688c4605