Frameworks: Slightly refactor Parcel code

Decrease the amount of code covered by a lock in Parcel. This allows
fetching multiple classes in parallel, with the slight downside that
these classes might be the same.

Reduces contention samples for this method in profiling by two thirds.

Test: m
Test: manual profiling
Change-Id: I96415659f6625df25800c60dd3bee1094255fcc3