Optimize Fragment operations so that minimal work is done.

Bug 29631389

When multiple operations are executed at once, they sometimes
cancel each other. For example, if the following transactions
are queued:

Transaction 1: add A
Transaction 2: replace with B

This can be trimmed down to add B.

This CL optimizes fragments in both add and pop directions.

Developers can choose not to allow optimization by
using FragmentTransaction#setAllowOptimization

Test: If6637e9f1c2a414bebaff6404efc45dd828378ad

Change-Id: Iab75be3e0aa388fc79b794e647ac6893165bebd7