Remove explicit creation of SurfaceSession in WM shell

Because it is a stateless connection object to SF, it is unnecessary
to have individual connections from each functions.

There were already several places use null for SurfaceControl.Builder,
so the singleton instance SurfaceComposerClient::getDefault() is
already in use. Then just use that one. That reduces unnecessary
IPC to SF when creating surface.

Bug: 163976519
Flag: EXEMPT reduce unnecessary invocation
Test: atest SplitTransitionTests
Change-Id: I4750ab7dc6e8544e2060ec89207f09f1241966ad