NetworkCapabilities: fix describeImmutableDifferences

This patch fixes the mask used in describeImmutableDifferences which did
not correctly turn NET_CAPABILITY_NOT_METERED into bit flag.

Bug: 63326103
Test: added unit tests, runtest frameworks-net
Change-Id: Ib6b390b1daef5912859302692af7dcd6cfd3e39a