NsdManager: remove duplicated argument validation

This patch simplifies argument validation in NsdManager public api and
regroup duplicated validation into common methods.

This makes stack traces more actionable as now specific errors will
cause the api to throw exception from specific methods, whereas before
IllegalArgumentException would be thrown from inside the same api method
for different reasons.

This patch also includes a couple of other small cleanups.

Test: $ runtest -x frameworks/base/tests/net/../NsdManagerTest.java
Bug: 37013369
Change-Id: Iaad13e13976e9bf8f508d7188f823f8184ac414b