Add function to query ADB state

This is what the USB manager will use to query the current ADB state in
the future.

Bug: 63820489
Test: make
Change-Id: I70484f97e56af4e5104611dc3b43ef83885671d7

core/java/android/debug/AdbManagerInternal.java

@@ -36,4 +36,9 @@ public abstract class AdbManagerInternal {
      * @param transport previously-added ADB transport interface to be removed
      */
     public abstract void unregisterTransport(IAdbTransport transport);
+
+    /**
+     * Returns {@code true} if ADB debugging is enabled.
+     */
+    public abstract boolean isAdbEnabled();
 }

services/core/java/com/android/server/adb/AdbService.java

@@ -63,6 +63,11 @@ public class AdbService extends IAdbManager.Stub {
         public void unregisterTransport(IAdbTransport transport) {
             mTransports.remove(transport.asBinder());
         }
+
+        @Override
+        public boolean isAdbEnabled() {
+            return mAdbEnabled;
+        }
     }
 
     private static final String TAG = "AdbService";
@@ -70,6 +75,8 @@ public class AdbService extends IAdbManager.Stub {
     private final Context mContext;
     private final ArrayMap<IBinder, IAdbTransport> mTransports = new ArrayMap<>();
 
+    private boolean mAdbEnabled;
+
     private AdbService(Context context) {
         mContext = context;