Close BluetoothGatt after disconnect. (#382)

This would cause issues on some readers with some devices. Left previous connection open and was prone to further unexpected communications.

identity-android/src/main/java/com/android/identity/android/mdoc/transport/GattClient.java

@@ -547,6 +547,7 @@ void drainWritingQueue() {
             Logger.d(TAG, "Chunk is length 0, shutting down GattClient");
             try {
                 mGatt.disconnect();
+                mGatt.close();
             } catch (SecurityException e) {
                 Logger.e(TAG, "Caught SecurityException while shutting down: " + e);
             }