NsdService: do not use ContentResolver directly

This patch changes NsdService to call registerContentObserver in the
ContentResolver class indirectly through NsdSettings.

This allows to easily intercept it and mock it in unit tests, and solves
test failures on the internal master branch where
registerContentObserver uses final or static methods that cannot be
worked around.

Bug: 32561414
Bug: 62044295
Test: runtest -x frameworks/base/tests/net/../NsdServiceTest.java
Change-Id: If4deb106de551746babb70196b20f21ece478850