Check to start the PiP animation early.

Before Android Q, we will start PiP animation when activity requests to
enter pinned stack. Since we will move the pinned stack control to SystemUI,
the original transition sequence would break and cause flicker.
An easy way to prevent flicker is to check if the animation needs to
start early.

Fix: 146432422
Test: Manual enter PiP with gesture/3-button navigation several times.
Test: atest PinnedStackTests
Change-Id: I646efe7fbf0c58224d55112ab1bc8749c3dc41da