Postpone IME show animation till IME is drawn in WM.

With this change, we postpone IME show animation until IME layout is fully
drawn in WM.
Earlier, we were showing IME right after InputMethodService called
Window.show(), which didn't guarantee that layout draw was completed.

Fix: 139487540
Bug: 111084606
Test: Manually verified as follows:
     1: Kill IME process or restart system to make sure IME window isn't
     created.
     2. Tap on edit field to make sure IME shows-up first time.
Change-Id: I35f3fdca43e6bae598a08237ba724962d575cb80