Add executing thread check in LegacyNetworkActivityTracker

One of the reasons for having locks in LegacyNetworkActivityTracker is
mNetworkActive is updated from non handler thread.
aosp/2606673 moves network activity change processing to handler thread.
All the methods that update mNetworkActive and mActiveIdleTimers should
be called from handler thread now.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I5c8cd0b253a77a2740ade3a0145ef80336b3b133