Use an ArrayMap instead of HashMap to track tether interfaces

( cherry-pick of 36f6f4e3 )

Since the number of tetherable interfaces should be small (e.g. <10),
the computational cost of a looking up objects in an ArrayMap is negligible,
and the memory required is less than a HashMap.

While here, remove some unused imports.

Change-Id: I3fb7091cfb93697d2fc6388b843d60f7e8e7e1a6
Test: Compiles.