docs: Added desc. of launcher app criteria in Q

As of Android Q, an app is included in the return value of
LauncherApps#getActivityList() unless it fulfills specific criteria.
This CL describes the conditions necessary for an app's activities
to *not* appear in the list.

Test: make ds-docs -j32

Bug: 130352392
Change-Id: Iad888a7a7f47d090da1b143ac152705577144d20