Mock the startActivityAsUser to prevent silent crash

**Root cause**
With b/351856795, we call startActivityAsUser instead of startActivity,
however we didn't mock the startActivityAsUser, hence it tried to start
the EditShortcutsActivity with invalid arguments which causes the
Settings app silently crashed in test

Bug: 366358444
Test: atest MenuViewLayerTest. Verify test passed and there is no slient
crash in the test's device log
Flag: EXEMPT bugfix

Change-Id: If60eba1e43c79a1a1e4a1e317e1420b182f68421