Revert isDefaultNetworkActive behavior when there is no default network

aosp/2605757 updated isDefaultNetworkActive returns false when there is
no default network.
But this change was not merged to U release branch and this change was
not well tested by dogfooding.
So this CL reverts the isDefaultNetworkActivity behavior change.

Bug: 279380356
Bug: 291870075
Test: atest FrameworksNetTests
Change-Id: Id88662faea9eeaba93d59ab2729f6204a3631ab1