Always update wallpaper surface scale from explicit request

This combines the logic of setting wallpaper scale from [1] and [2].
So updateScaleIfNeeded is skipped in prepare surface if the window
is invisible-requested, but if someone calls setWallpaperZoomOut,
it can ensure the latest scale is applied to the surface. That avoids
showing an intermediate scale:
   Apply scale 0.8
   Request scale 0.9
   Wallpaper becomes completely invisible (not apply 0.9)
   Wallpaper becomes visible
   Request scale 1.0
   Scale jumps from 0.8 to 1.0.

[1]: I8e8e21fcd3b8f4ff0f9b22d04c1abb908e719422
[2]: I1db9207c79bc45dedd90e7b13af5517f09abe4a7

Bug: 416216706
Flag: EXEMPT bugfix
Test: Launch app from home and observe wallpaper scale.
Change-Id: Id192ee94610f48661f92ae1d17a18d62dea35197