Replace user with debuggable check in ConnectivityService

IS_USER is a hidden symbol, whereas IS_DEBUGGABLE is part
of the module API.
The check is generally equivalent, as non-user builds are eng or
userdebug, which are debuggable.

Bug: 170598012
Change-Id: I7d09939652844eb2e3a6f0200b1ba50691a147da
Test: atest FrameworksNetTests