Add null check to VR ensureNoVrActivitiesDisplayed

It turns out that UiDevice.getCurrentPackageName can return null, so
handle that case to avoid NullPointerExceptions.

TBR=mthiesse@chromium.org

Change-Id: I8cb220d5df1d0f110b2bded7473f20ba7d859872
Reviewed-on: https://chromium-review.googlesource.com/1099838Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567010}

chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/util/VrTestRuleUtils.java

@@ -105,7 +105,8 @@ public class VrTestRuleUtils {
      */
     public static void ensureNoVrActivitiesDisplayed() {
         UiDevice uiDevice = UiDevice.getInstance(InstrumentationRegistry.getInstrumentation());
-        if (uiDevice.getCurrentPackageName().contains("vr")) {
+        String currentPackageName = uiDevice.getCurrentPackageName();
+        if (currentPackageName != null && currentPackageName.contains("vr")) {
             uiDevice.pressHome();
             // Chrome startup would likely be slow enough that this sleep is unnecessary, but sleep
             // to be sure since this will be hit relatively infrequently.