[Android WebAPK] Fix showing lightweight FRE on WebAPK launch

Previously, if a user:
- with the --enable-lightweight-fre command line flag
- who has not previously gone through the first run experience
tried to launch a WebAPK the WebAPK did not launch and the FRE was not
shown.

This was occurring due to Intent.FLAG_ACTIVITY_NEW_TASK not being set
as part of the FRE launch intent. (The caller is
WebappLauncherActivity).

This CL:
- Makes LightweightFirstRunActivity "singleInstance" like
FirstRunActivity
- Removes LightweightFirstRunActivity-only logic which finishes the
current
  LightweightFirstRunActivity if one is already running

BUG=738133

Change-Id: Ifff19eb505f91efa5ea38ba2069f6159be7e7d4f
Reviewed-on: https://chromium-review.googlesource.com/563864Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486840}

chrome/android/java/AndroidManifest.xml

@@ -409,7 +409,7 @@ by a child template that "extends" this file.
         </activity>
         <activity android:name="org.chromium.chrome.browser.firstrun.LightweightFirstRunActivity"
             android:theme="@style/SimpleDialog"
-            android:launchMode="singleTop"
+            android:launchMode="singleInstance"
             android:autoRemoveFromRecents="true"
             android:windowSoftInputMode="stateHidden|adjustPan"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|mcc|mnc|screenLayout|smallestScreenSize">

chrome/android/java/src/org/chromium/chrome/browser/firstrun/FirstRunFlowSequencer.java

@@ -376,17 +376,10 @@ public abstract class FirstRunFlowSequencer  {
                 List<WeakReference<Activity>> activities = ApplicationStatus.getRunningActivities();
                 for (WeakReference<Activity> weakActivity : activities) {
                     Activity activity = weakActivity.get();
-                    if (activity == null) {
-                        continue;
-                    } else if (activity instanceof LightweightFirstRunActivity) {
-                        // A Generic or a new Lightweight First Run Experience will be launched
-                        // below, so finish the old Lightweight First Run Experience.
-                        activity.setResult(Activity.RESULT_CANCELED);
-                        activity.finish();
-                        continue;
-                    } else if (activity instanceof FirstRunActivity) {
+                    if (activity instanceof FirstRunActivity
+                            && !(activity instanceof LightweightFirstRunActivity)) {
                         isGenericFreActive = true;
-                        continue;
+                        break;
                     }
                 }