Add idleTimer existence check before removing idleTimer

IdleTimer tracking can be disabled if configured timeout is 0 or
negative value.
In this case, setupDataActivityTracking does not add idleTimer and
removeDataActivityTracking causes NPE.
This did not cause crash since the code to cause NPE is wrapped by try
catch.
This CL adds check to avoid causing NPE.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: Id774c5ee3d0318d9736f287c50ccf4c7c5743aef