Move activity starting logic into new ActivityContext extension.

This code branch is needed for taskbar, so needs to be moved from the
activity classes to an interface that both can share.

Test: Manual
Bug: 216683257
Change-Id: I042c0dcf08207d94847b9d7a1783acc14613ef5f
diff --git a/src/com/android/launcher3/Launcher.java b/src/com/android/launcher3/Launcher.java
index 5c406bd..2f9b563 100644
--- a/src/com/android/launcher3/Launcher.java
+++ b/src/com/android/launcher3/Launcher.java
@@ -2065,7 +2065,7 @@
 
     @TargetApi(Build.VERSION_CODES.M)
     @Override
-    protected boolean onErrorStartingShortcut(Intent intent, ItemInfo info) {
+    public boolean onErrorStartingShortcut(Intent intent, ItemInfo info) {
         // Due to legacy reasons, direct call shortcuts require Launchers to have the
         // corresponding permission. Show the appropriate permission prompt if that
         // is the case.