Optimizing: Clean up after const-string sharpening.

Do not look up the String for JIT, just check if it's in the
dex cache. Strings on hot paths should already be resolved
and we don't want to unnecessarily increase JIT compile time
to have a chance of improving a cold path.

Also, change the enum LinkerPatchType to be an inner enum
class of LinkerPatch and clean up casts between pointers and
uint64_t.

Change-Id: Ia6e0513af1a84ce94a3b30edac0c592157d374ec