Remove unused testutils lib from tethering tests

frameworks-base-testutils is unused in tethering integration tests, so
the dependency can be removed.
That test library also contains test classes, so removing the dependency
allows tethering tests to stop running the associated tests.

Also add jarjar rules to the unit tests to zap (remove) the test classes
from the output APK. Ideally the unit tests should stop depending on
that library too (TestableLooper can be used instead of TestLooper), or
the frameworks-base-testutils library should stop including test
classes.

Bug: 167968946
Test: m CtsTetheringTest TetheringTests
Change-Id: Ie950ce5b6181e388a7f71da430fd2dd8ada32e49