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.

(Cherry pick from master cadb9498)

Bug: 159709775
Fixes: 159709775
Test: wembley-userdebug boots and has functioning Launcher
Change-Id: I17904f9ad88abc66364675426545eae423cb3175
Merged-In: I17904f9ad88abc66364675426545eae423cb3175