Fix a bug about the z-order of layers caused by merging transactions

Each WindowContainer has its own pending transaction. These transactions
may be merged to the global one within WindowContainer.prepareSurfaces()
in a hierarchy-based order, not in a time-based order. It may cause
that a later surface operation overwritten by an earlier surface
operation. For example, atokenA.setLayer(t1, 0) was called earlier, and
then atokenA.setLayer(t2, 1) was called later. However, the layer of
atokenA might eventually be 0 because t1 could be merged into the
global transaction later.

This CL uses a single transaction per display to solve this problem.

Fix: 120282809
Test: atest SurfaceAnimatorTest
Test: Manual test with the steps in the issue.
Change-Id: Idca57d01d8be884369510642c2d9345b6ee4e3b1