Don't enable DscpPolicyTracker on pre-T devices.

The code cannot work before T because before T the tethering
module can only load BPF programs/maps into /sys/fs/tethering/bpf
and the system server cannot access that directory.

Additionally, this is causing unit tests to fail on pre-T devices
because on those devices the seccomp filter blocks the bpf
syscall from apps (including Java tests).

Test: revert aosp/1907693 and atest FrameworksNetTests:ConnectivityServiceTest
Change-Id: I6c398031dffb840da1d723b11ea4d0845ad6c6f2

service/src/com/android/server/ConnectivityService.java

@@ -1495,7 +1495,14 @@ public class ConnectivityService extends IConnectivityManager.Stub
                 mLingerDelayMs, mQosCallbackTracker, mDeps);
 
         try {
-            mDscpPolicyTracker = new DscpPolicyTracker();
+            // DscpPolicyTracker cannot run on S because on S the tethering module can only load
+            // BPF programs/maps into /sys/fs/tethering/bpf, which the system server cannot access.
+            // Even if it could, running on S would at least require mocking out the BPF map,
+            // otherwise the unit tests will fail on pre-T devices where the seccomp filter blocks
+            // the bpf syscall. http://aosp/1907693
+            if (SdkLevel.isAtLeastT()) {
+                mDscpPolicyTracker = new DscpPolicyTracker();
+            }
         } catch (ErrnoException e) {
             loge("Unable to create DscpPolicyTracker");
         }