Instead of returning a bare jobject, return a const reference to a
JavaRef, making it clearer that the caller does not receive ownership.
Most uses pass the result directly to a Java method; update the cases
which store it in a variable or pass it elsewhere.

Also, clean up the code surrounding the GetApplicationContext calls a
bit:
- There's no need to DCHECK this or AttachCurrentThread as the
  implementations already do that internally
- Change method parameters that take contexts as jobject to JavaRef, and
  also other parameters of those methods where reasonable.

BUG=506850

Review-Url: https://codereview.chromium.org/2231923002
Cr-Commit-Position: refs/heads/master@{#412219}