Disable "MissingClass" lint check in QuickstepResLib

QuickstepResLib just contains resources. The code implementation for custom views is in NexusLauncher, so the linter doesn't know about this and falsely thinks a custom view resource is added without a corresponding class.

Change-Id: Icfe57c82af679c22428319e20d4c0e29b5c374fe
Fix: 383318069
Bug: 376817827
Flag: EXEMPT trivial build config change
Test: Add a custom view resource to QuickstepResLib, add code for custom view to NexusLauncher, run presubmits, check lint warning isn't there