Add comment for NetworkStack permissions

Clarify that permissions must not be in the base package to avoid
crashes on devices using a network stack running in the system server
process.

Test: m
Change-Id: Ib13df84a43e960d01217c208d1c6775d1f81edc4
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 9b60dc3..0fb342c 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -21,6 +21,10 @@
           android:sharedUserId="android.uid.networkstack">
     <uses-sdk android:minSdkVersion="28" android:targetSdkVersion="28" />
 
+    <!-- Permissions must be defined here, and not in the base manifest, as the network stack
+         running in the system server process does not need any permission, and having privileged
+         permissions added would cause crashes on startup unless they are also added to the
+         privileged permissions whitelist for that package. -->
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />