Fix trivial warnings in Tethering.java

Add some missing @Override annotations.
Use template arguments to avoid explicit casting.
Ignore unclosed IndentingPrintWriter in dump().

Bug: 28798823
Test: Compiles.

Change-Id: I2cb6eb384ca23057c8059f4842b1c0d8248d03f8
(cherry picked from commit 6d475970)