Add explicit android:exported="true" declarations

We add this for GestureSandboxActivity and
com.android.launcher3.uioverrides.QuickstepLauncher to
reinstate the behavior of go/ag/11746107 which was
incorrectly reverted in go/ag/11963698.

We also add this for HotseatEduActivity which was
newly added in go/ag/11963698.

Bug: 159709775
Test: wembley-userdebug boots and has functioning Launcher
Change-Id: I17904f9ad88abc66364675426545eae423cb3175
diff --git a/quickstep/AndroidManifest-launcher.xml b/quickstep/AndroidManifest-launcher.xml
index 60afddb..1a634ce 100644
--- a/quickstep/AndroidManifest-launcher.xml
+++ b/quickstep/AndroidManifest-launcher.xml
@@ -53,7 +53,8 @@
             android:resizeableActivity="true"
             android:resumeWhilePausing="true"
             android:taskAffinity=""
-            android:enabled="true">
+            android:enabled="true"
+            android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.HOME" />