Move NetworkAgent#register to a new method.

Calling IPC in a constructor is unusual and confusing, and can be
considered bad form. There are multiple reasons for this :
• Users can't obtain an instance of the class without calling the
  constructor, but they can't always afford an IPC where they need
  this, forcing them to know about the implementation detail and
  sometimes design around it.
• On a related but generalized note, constructors should usually
  be fast for the same range of reasons.
• Having a separate method to register the agent simply gives more
  flexibility to the app.
• It's also a lot easier to test.
But also we can't fix it without an update to the API, so here it is.

Another reason for doing this is consistency with the NetworkProvider
API.

Bug: 138306002
Bug: 139268426
Test: atest FrameworksNetTests FrameworksWifiTests FrameworksTelephonyTests
Change-Id: I1ee5c7b1353d581e487c8a8a159009bebd781643
Merged-In: I1ee5c7b1353d581e487c8a8a159009bebd781643
(clean cherry-pick from internal branch)