ConnectivityServiceTest: fix flaky testNetworkRequestMaximum

Registered requests are not keyed by PendingIntents in
ConnectivityService, which means that unregistering a request with a
PendingIntent causes a linear search in all registered requests.

testNetworkRequestMaximum was registering too many PendingIntents
simultaneously, causing the unregistration loop to have n^2
complexity and to take a long time to take effect.

To make the unregistering loop less likely to trigger a timeout on
waitForIdle, this patch changes the test to not register MAX_REQUEST
number of PendingIntent, but instead mixes a small number of
PendingIntents with NetworkCallbacks to reach MAX_REQUEST number of
simultaneously registered requests.

When unregistering these requests, callbacks are unregistered first.

Bug: 32561414
Test: runtest frameworks-net
Change-Id: I48b882c884abe20b388190b7f28baee293446f37