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();
 }