[Flexiglass] Simplify NSSL's stack height calculation

Separate the Flexiglass and the Legacy way of calculating the stack
height. When Flexiglass is enabled:
 - verify that we never add listeners to
   NSSL.mStackHeightChangeListeners
 - stop setting NSSL.mContentHeight, and use mIntrinsicContentHeight
   instead
 - don't use NSSL.getIntrinsicPadding()
 - don't use NSSL.getScrimTopPadding()

Bug: 332574413
Fixes: 347768714
Test: manually verify if the stack height is updated correctly in the
shade, and in the lock screen
Flag: com.android.systemui.scene_container

Change-Id: Ic05953a85f7c8b3855eea5e2c8a04f2ea8445cd8