Only allow system or sysui to set launch activity type

Or starting assistance type activity from assistant is also
allowed.

Otherwise, apps can only start standard activity or other type of
activities based on the given intent (such as home activity).

Bug: 185872145
Test: atest StartActivityTests
Test: atest AssistantStackTests

Change-Id: Iba1d19f1ac5e9008580c96a556f3eeab81f112fe