Fix FieldGap priority queue ordering bug.

The priority queue for keeping track of gaps when packing fields in a
class object had the order reversed, giving priority to smaller gaps
instead of priority to larger gaps. This led to cases where fields
were not placed in gaps when they could be.

Bug: 22460222
Change-Id: I062e772e030c034adc227d75deed31c3322e203e
8 files changed