Change android.permission.SERIAL_PORT to signature|system.

Bug: 7367956
Change-Id: I0bc65936ba59c43c18d9a6ba52affff4c921f2fb

Author: gcondra

core/res/AndroidManifest.xml

@@ -2019,7 +2019,7 @@
     <permission android:name="android.permission.SERIAL_PORT"
         android:label="@string/permlab_serialPort"
         android:description="@string/permdesc_serialPort"
-        android:protectionLevel="normal" />
+        android:protectionLevel="signature|system" />
 
     <!-- Allows the holder to access content providers from outside an ApplicationThread.
          This permission is enforced by the ActivityManagerService on the corresponding APIs,