Specify the default theme for splash screen window.

By default if application didn't specify a theme, PhoneWindow will
get the default theme by Resources#selectDefaultTheme for itself
and currently it could be Theme_DeviceDefault_Light_DarkActionBar,
which will shows an ActionBar on splash screen window, and looks
bad for starting window.
For this case, use Theme_DeviceDefault_DayNight for splash screen
window should be a better choice.

Bug: 73289295
Test: atest StartingSurfaceDrawerTests
Change-Id: Ia47afcb26c7aaa7781a24e3fe8122e58901776dc