Fix a crash when changing preferences

The crash occurs when some app has more than half its limit
in requests that will need to be moved to some other default
network upon changing the preferences.
This will send the requests for this app over the limit
temporarily when creating new requests for the reevaluated
ones.

While ConnectivityService has a provision for making a
transaction-like addition/removal of requests that is meant
to avoid exactly this kind of crash with the transact()
method on PerUidCounter, the code only transacts on
mSystemNetworkRequestCounter. But these requests are counted
in the mNetworkRequestCounters, which is not part of the
transaction, causing the crash anyway.

To avoid the problem, this patch allows the request counters
to go over the max if and only if the system server is
updating the request counts for a UID other than its own.
This should allow only the case where ConnectivityService is
moving the requests over to the new per-uid default, while
keeping the exception when registering from an app (then the
calling UID is not the system server), or when the system
server registers its own requests (then the UID inside the
request is that of the system server).

A much better solution than this patch would be to completely
eliminate the transact() method by somehow unregistering the
old ones before creating the new ones.
However this would be a much bigger and difficult patch than
this, and much more dangerous, because callers depend on the
list of requests to find out the old requests to remove, so
they have to be created first.

Another possible clean solution would be to count the
requests not in the NRI constructor, but later. This would be
more error-prone though because it would be very easy to
create an NRI without counting it.

Bug: 192470012
Test: ConnectivityServiceTest. Improve tests so they catch
      this case.
Change-Id: Ia482e6fbf2bf300ce6cbaca72810d394ed201b98

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