Annotate ExitPipToAppViaExpandButtonTest

Annotate ExitPipToAppViaExpandButtonTest as Flaky while we are
investigating the underlying issue of it

- The behind Activity flashes for one frame at the end of expanding PiP
  to full-screen transition

Flag: TEST_ONLY
Bug: 391734110
Test: atest ExitPipToAppViaExpandButtonTest
Change-Id: Iad6eeb895ae2b517facc64ccb034b2609dd4288d