Set mEnterAnimationPending to true if a window was not visible

mEnterAnimationPending will be cleared when enter-animation is applied.
However, if a window was already visible, the enter-animation won't be
applied. This CL ensures mEnterAnimationPending is set to true only if a
window was not visible to prevent the state of mEnterAnimationPending
from being leaked.

Bug: 128510422
Test: Manual:
1. Go to Settings > Accessibility > Magnification
   > Magnify with triple-tap.
2. Turn on "Use service".
3. Triple-tap.
4. Press [back] button.
5. See if the orange border is around the entire screen except the
   navigation bar.
Note: 1. ~ 4. should be done within 10 seconds.

Change-Id: Id1a811175214af25ec8014b3f5814cff16d70331