Fixed incorrect behavior of isDataConnectivityPossible

When the device is on single PDN network, isDataConnectivityPossible
returns incorrect value due to the wrong logic in
DataNetworkController.evaluateNetworkRequest(). Fixed by adding
existing network satisfying check and network request priority
check.

Bug: 259911671
Test: atest ConnectivityManagerTest
Change-Id: I6b073f730282ca68311b4407936871e65663cc7a