Add final modifier to private static DEBUG constants

Marking final allows the gated debug code (generally logging) to be
stripped from production builds. This saves ~20KB from system server.

Follow-up work will explore presubmit guards for similar DEBUG cases
(including guarding against DEBUG = true).

Bug: 203143243
Test: m
Change-Id: Ic87a8d8bf183ac0b36c790a2f4a4661ae5e1a87f