Move network activity change processing to handler thread

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

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I2598feef93ef34e10bf576560f687ff7fc449ab9
1 file changed