Don't tear down even when no requests

Whether tear down a network due to requests should be a decision made by connectivity service. Connectivity service will linger for a while before unwant the network.
The change is to address the issue where there is a setup
request/reponse delay, so the request brought up the network is release before the setup reponse, led to the network marking the data profile as throttled, which blocks future requests to be satisfied.

Fix: 331301784
Test: voice call + data browsing
Test: atest tests/telephonytests/src/com/android/internal/telephony/data/DataNetworkTest.java

Change-Id: I184f3042fe0450a74a746fa1d7cd98a0490ce6e7