Move the Once cache for dexpreopt.GlobalConfig into the dexpreopt

package.

Preparation for a future CL that will need to get the make-written
GlobalConfig from within dexpreopt.

Also rename the Load*Config functions to Parse*Config, since they don't
actually load the config files anymore.

Bug: 145934348
Test: m
Change-Id: I71df11c1e042ca8135d273a7263e9539ea3cd68f