DO NOT MERGE Ignore DUN in describeImmutableDifferences

This patch changes describeImmutableDifferences in NetworkCapabilities
to ignore differences in NET_CAPABILITY_DUN, so that updateCapabilities
in ConnectivityService to not report wtf errors when a NetworkAgent
degrades its NetworkCapabilities object by removing NET_CAPABILITY_DUN.

Bug: 65257223
Test: runtest frameworks-net
Change-Id: I115ed1b366da01a3f8c3c6e97e0db8ce995fd377