Combine class sets when writing a class table

Reduces the boot.art size since we allocate a new class set with the
default load factor. The load factor in the combined class table is
higher to pruning classes, this reduces the size of the boot image.

Also fixes the hypothetical case where the class table being written
has multiple class sets.

Boot.art size
Before: 8122368
After: 8044544

Slight cleanup in patchoat.cc.

Bug: 22858531
Change-Id: I31b3ea2004ece6f6c853ee906074bb2792dee958