Fixes Context Hub feature naming and documentation

- Adds "_" in variable name for consistency with feature string
- Adds documentation on what the feature is used for

Bug: 149475852
Test: None
Change-Id: I6eca279df8409de1155cd7014647a705d0d31d6f

services/java/com/android/server/SystemServer.java

@@ -1741,7 +1741,7 @@ public final class SystemServer {
             mSystemServiceManager.startService(SensorNotificationService.class);
             t.traceEnd();
 
-            if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_CONTEXTHUB)) {
+            if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_CONTEXT_HUB)) {
                 t.traceBegin("StartContextHubSystemService");
                 mSystemServiceManager.startService(ContextHubSystemService.class);
                 t.traceEnd();