ObaStudyRequest.Builder.URI should be an instance field, not static

[aaronbrethorst]

Problem

In ObaStudyRequest.java:29, URI is declared private static, meaning it's shared across all Builder instances. If a Builder runs successfully for one region, then a subsequent Builder hits an early return (e.g., region == null or baseUrl == null), URI retains the stale value from the previous invocation. The next build() call would silently create a request targeting the wrong region.

Suggested Fix

Change URI from a static field to an instance field:

public static class Builder {
    private Uri mUri = null;  // was: private static Uri URI = null;
    // ...
}

Context

Discovered during review of #1525, which correctly added a null guard for getCurrentRegion() but the static field issue is pre-existing.