Get system_notification_accent_color from public resources.

LockdownVpnTracker gets system_notification_accent_color from
com.android.internal.R. This is not stable across releases, so it
can crash ConnectivityCoverageTests when run on older releases.

Use the system API constant instead, since that is guaranteeed to
be stable. The constant was added in ag/5795405.

Test: atest ConnectivityCoverageTests
Change-Id: I956f9fac4b89cf8429bf46682b52c0203cc4210e