Fix setCapabilities.

P introduced setSSID, UIDs and unwanted capabilities.
None of these exhibit commutative behavior through combineCapabilities
because their semantics don't allow it. Therefore
NetworkRequest.setCapabilities() is badly broken around any of
these. Look at the comments in the new tests to realize the
extent of the damage.

Clean cherry-pick of ag/4083952

Bug: 79748782
Test: new tests written, old tests pass
Change-Id: Iafe074126132a82af37f4bf056c4a7b8d56bdc83
Merged-In: Ia5bebf8a233775367bbf1b788870528934ecbcfb
Merged-In: I13d7782a6c0c7b1f94137995bbb0d257a58d89c1