Polish non-main app window flashing under starting window.

Skip playing entering animation for non-main app window if starting
window existing, the enter animation can shift window position, so
it may seems blured during fade-out snapshot starting window.

Preventing from apply dim layer for starting window, the dim layer
should be used for other app window, but it is impossible to place
under starting window.

Bug: 321874837
Test: follow issue description, launch app from home several times.
Change-Id: I1b5bf7ae33fd19f7cf37d38902aab41ffb5b10f5