Revert "Access UpdateManager via getInstance()."

This reverts commit c2b04a4c.

Reason for revert: Caused Cast build break

Original change's description:
> Access UpdateManager via getInstance().
> 
> This API changes in AT O.
> 
> Bug: internal b/77915277
> Test: Compile
> Change-Id: Id3c651378d9ac12319cd5aa437e9c56d48ea3096
> Reviewed-on: https://chromium-review.googlesource.com/1033472
> Commit-Queue: Doug Horn <doughorn@google.com>
> Reviewed-by: Luke Halliwell <halliwell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#558312}

TBR=halliwell@chromium.org,doughorn@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: internal b/77915277
Change-Id: I9bdb2c5c4a7c841fe26b2b5d212ebb32c6a544a2
Reviewed-on: https://chromium-review.googlesource.com/1059952Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558807}

chromecast/browser/android/apk/src/org/chromium/chromecast/shell/CastSysInfoAndroidThings.java

@@ -14,8 +14,10 @@ import org.chromium.base.annotations.JNINamespace;
  */
 @JNINamespace("chromecast")
 public final class CastSysInfoAndroidThings {
+    private static final UpdateManager sUpdateManager = new UpdateManager();
+
     @CalledByNative
     private static String getReleaseChannel() {
-        return UpdateManager.getInstance().getChannel();
+        return sUpdateManager.getChannel();
     }
 }