[Android] Destroy mSetActivity in ChromeActivityTestRule

This CL destroys mSetActivity in rule teardown. The activity is
launched manually inside the test rule and it is not ended prior to
this change.

This is necessary to avoid crash once we will teardown the
SigninTestUtil properly.

Bug: 1083138
Change-Id: Ibacf4a78c22b5316983a3ae208070c075b042738
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202796Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770559}

chrome/test/android/javatests/src/org/chromium/chrome/test/ChromeActivityTestRule.java

@@ -140,6 +140,10 @@ public class ChromeActivityTestRule<T extends ChromeActivity> extends ActivityTe
     @Override
     protected void afterActivityFinished() {
         try {
+            if (mSetActivity != null) {
+                // Destroy the activity launched manually in this test rule.
+                ApplicationTestUtils.finishActivity(mSetActivity);
+            }
             ApplicationTestUtils.tearDown(InstrumentationRegistry.getTargetContext());
             Thread.setDefaultUncaughtExceptionHandler(mDefaultUncaughtExceptionHandler);
         } catch (Exception e) {