Enable two-part fade out for splash screen reveal anim

ag/24465007 provided a FADE_OUT splash screen anim option
to replace the default 'reveal'.

This change enables the use of the existing configs supporting
a two-part fade where the icon/branding views and the splash
screen itself can be faded indepedently.

Take the following config from Wear for example:

<integer name="starting_window_app_reveal_icon_fade_out_duration">50</integer>
<integer name="starting_window_app_reveal_anim_delay">50</integer
<integer name="starting_window_app_reveal_anim_duration">200</integer>

This animation will fade out the icon (50 ms), at the same time waiting
to start the splash screen animation, which will then fade out over
200ms (total duration 250ms)

Note that the original fade out was implemented by TV - as such config
values have been put in place to make this change a no-op with respect
to that usage.

Bug: b/286280723
Test: manual validation of app launches with appropriate configs
Change-Id: I74b8b0b37538187f80d40c8561a98dac0f4eaffa