Start listening to telephony changes in #start()

Starting to listen on the handler races with the rest of
the constructor. The class already has a #start() method
meant to start listening, so implement better practices
by leveraging this method.

Test: CtsNetTestCases
      FrameworksNetIntegrationTests
      FrameworksNetTests
Change-Id: I772f761d3ca5f9711b9d988e6330b0878814f491
2 files changed