Wrap call to updateLayoutEngine with dreamsV2 flag

Fix a previous change that is not guarded with the right flag.
Only when flag is enabled, listen to configuration change and
call updateLayoutEngine.

Bug: b/378163332
Fixes: b/378163332
Flag: android.service.dreams.dreams_v2
Test: atest ComplicationHostViewControllerTest
Change-Id: I2da71debb557f1b564bdcd941f2fe8c80624971c