Add descriptions to compat changes.

Capture the comment above a definition of a compat change and make it
the description.
Next: make sure existing changes use supported format (only /**
comments, only above the annotations), and use in developer UI.

Bug: 144927670
Test: atest com.android.server.compat.CompatConfigTest
Change-Id: Ib23f341baa171599654c351693e4b0ddf4b2515c