Remove empty default constructor

Unused. Initially added to ease mocking/inheritance for testing
purposes, but apparently not necessary because a package-private
default constructor is auto-generated.

Also make fields final.

Bug: 123600502
Test: atest cts/tests/tests/os/src/android/os/cts/BuildVersionTest.java
Change-Id: I123a19fd0760259db67344bb827c0cf2de3b85c3