Revert "[flexiglass] Verify that NSSL#mContentHeight is not accessed"

Partially revert submission 30042981 (remove calls to
SceneContainerFlag.assertInLegacyMode).

Reason for revert: Crashes Flexiglass

Reverted changes: /q/submissionid:30042981
Flag: EXEMPT removing assertions
Bug: 376018224
Test: Unit tests pass.
Test: Manually verified by opening the shade on lockscreen.

Change-Id: I9f12c27d3dfd5e461f90b23fc4fa307f8cbccee5