Remove layer from previous relative list.

If a layer is getting a new zOrderRelative, it needs to remove itself
from its previous zOrderRelative's list, if it had one.

Test: Open app that opens the ime. Switch to another app that opens the
ime. Relative layers are updated properly.

Change-Id: I54fbfc0f926d39a1630f3b2faed04fbf428b4fe3