ART: Make the PathClassLoader fast-path recursive

Allow an arbitrary nesting of path class-loaders. This will also
handle the fake java.lang.BootClassLoader as a class-loader object
correctly (which is of some importance to compile-time initialization).

Bug: 19781184
Bug: 19542228
Change-Id: I61f0249cf4ec8df08a83ccbd29bcf067619c28c0