Reduce unnecessary invocation of shouldImeAttachedToApp

Is is more common that IME is invisible. And the visibility check
can be 10 times faster than shouldImeAttachedToApp.

The method needsRelativeLayeringToIme may be called hundreds of times
for assigning layer during launching an app.

Bug: 297502610
Flag: EXEMPT optimization without logic change
Test: ZOrderingTests# \
      testAssignWindowLayers_ForImeWithAppTargetWithChildWindows
Change-Id: I1ddcd07c7d5e7651580bfe76c487a4a6f56b5e76