Strip verbose logs in SystemUI

This change adds rules to proguard.flags to strip Log.v and Slog.v in
SystemUI. This results in a 37.5KiB reduction in dex size.

Stripping will only occur in user/userdebug builds; eng builds will not
be affected

Test: Build; check before/after .apk using ApkDiffStats.
Bug: 223674938

Change-Id: I203c589ea8a856bf3dc01bc876dbd2231c336d41