Starting window performance tuning.(8/N)

Create splashscreen thread with priority -10 in shell for starting
window controller, the definition is moved from
AMS#TOP_APP_PRIORITY_BOOST to Process#THREAD_PRIORITY_TOP_APP_BOOST.

Create and set SplashScreenView object on the starting window after the
first simple view on the starting window drawn, in this way the
transition can be prepare early than wait the actually view was draw,
which can save some time.

Bug: 73289295
Bug: 131311659
Test: atest StartingSurfaceDrawerTests ShellTaskOrganizerTests
Test: atest SplashscreenTests
Change-Id: Ibb430a66d6b329e74d62de047fbf37205cb66a96