NetworkCapabilities: Unhide TRANSPORT_LOWPAN

This commit simply unhides the constant `TRANSPORT_LOWPAN` in the class
`android.net.NetworkCapabilities`, adding it to the public API.

Bug: b/63600204
Change-Id: I794635814c0bd3891cd22fcc1e31614e4d6ac654

core/java/android/net/NetworkCapabilities.java

@@ -421,7 +421,6 @@ public final class NetworkCapabilities implements Parcelable {
 
     /**
      * Indicates this network uses a LoWPAN transport.
-     * @hide
      */
     public static final int TRANSPORT_LOWPAN = 6;