Fixes isDefaultNetworkActive and onNetworkActive unreasonable behavior

This CL changes the behavior as follows.

Before this CL, isDefaultNetworkActive returns true if there is no
default network.
After this CL, isDefaultNetworkActive returns false if there is no
default network on all releases.

Before this CL, LegacyNetworkActivityTracker does not call
onNetworkActive callback for networks that tracker does not add the idle
timer to (e.g.  ethernet, cell network configured with activity tracking
disabled).
After this CL,
On T-, onNetworkActive keeps the current behavior.
On U+, onNetworkActive is called regardless of the idle timer existence. This behavior is consistent with ConnectivityManager#isDefaultNetworkActive, which always returns true for a network that has no activity tracking.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I7b1d493fea0ab028b53a3d640a58a00ebbdcc143