Add enter/exit animation style to prevew and mini keyboard

TODO This animation style API of PopupWindow can be called only in
Gingrebread or later.  We should refactor this using reflection to be
able to be compatible with Froyo.

Bug: 3004787
Bug: 2973989
Change-Id: I2abfcc853b1a0af2bccee7bdf5fbd723456b7d34