Fix the content of the bottom split being truncated while showing IME

Freeze the configuration size when shifting split layout so that client
apps can calculate insets properly and won't get config or relaunch.

Fix: 192410210
Test: atest WMShellUnitTests
Test: verified the content of the bottom split won't be truncated when
adjusting split layout with IME and won't get relaunch.

Change-Id: I38da063ffacc19b691c14b74d118949293414492
4 files changed