Fix NullPointerException in ConnectivityService

[Problem]
NullPointerException occurs in ConnectivityService when boot up.

[Root Cause]
When ConnectivityService starts up, isTetheringSupported may access
UserManager that has not been created yet. Therefore,
NullPointerException occurs.

[Solution]
Changed to create UserManager before makeTethering().

Bug: 134351774
Test: ConnectivityServiceTest
Change-Id: I299ecfa6af7295c1f3922f37d90eb5d3ab0d0286