Allow NetworkAgent "immutable updates" to NetworkCapabilities

This patch loosens the validation checks when a NetworkAgent updates it
NetworkCapabilities: instead of checking that capabilities labeled as
"immutable" stay identical across updates, it is now accepted to change
immutable capabilities in a way that the new NetworkCapabilities
satisfies the old NetworkCapabilities.

This allows a NetworkAgent to update itself in order to match more
requests, but will still catch NetworkAgents that sends degradation
updates causing potentially requests to not match anymore.

Bug: 64125969
Test: runtest frameworks-net
Change-Id: I2a1b3f9c0be6415e40edc989d0c1b03b5631f7b1