Draw splashscreen edge to edge

 - Switch from PhoneWindow to FrameLayout
 - Ensure the Action bar is not shown on the splashscree
 - Do not show the contrast scrim under the system bars
 - Use the latest insets API to layout fullscreen

Test: Updated CtsWindowManagerDeviceTestCases:SplashscreenTests
Bug: 181852475
Bug: 184941669
Change-Id: Ifc04bd2227394415cc74c20d2974f4726ec9c789