[flexiglass] Fix NSSL alpha during HUN

This CL also renames `maxAlphaForExpansion` to `maxAlphaForKeyguard` since this name was very misleading, and was based on an incorrect assumption that this alpha was supposed to be used by expansion.

This also adds a dump of the NSSLController's alpha source, and when debugging, logs changes to the alpha and source.

Finally, this disables a few calls into NSSLController from legacy code, now that they are bound.

Fix: 328244289
Test: atest SystemUITests
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I9a9b3734a4a502bf4a6b2a3cd4baffd5f5646e4b