Remove locks from LegacyNetworkActivityTracker

Following CLs update LegacyNetworkActivityTracker to support multiple
network activity tracking on U+ devices.
This CL is a preparation for them.

Reasons for having locks in LegacyNetworkActivityTracker were 1)
mNetworkActive can be updated from non handler thread 2)
isDefaultNetworkActive can be called from non handler thread and this
return boolean based on mNetworkActive and mActiveIdleTimers.

aosp/2606673 moves the activity change processing to handler thread
and resolved the first reason.
This CL updates isDefaultNetworkActive to just return
mNetworkActive and resolved the second reason.

So, now LegacyNetworkActivityTracker doesn't need locks and this CL
removed the locks.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I12e3a00c6f8c4a0c40b45b9461860fe2e82fe22a