Create a Builder for Ringtone, add unit tests.

Currently hidden until the full new API is ready, but convert private
factory methods from RingtoneManager to use it. This moves towards
making the Ringtone class less mutable, and particularly for all required
media player properties (esp AudioAttributes) being available for the
initial construction.

Adds an initial unit test covering basic properties and fallback, and
"documenting" what happens when the uri is null, which was unclear before.

Bug: Bug: 261571543
Test: manual, presubmit
Change-Id: I81d512e9b21b64a2cca0450cb85c52d8a2125865