docs: Updated ShortcutManager API reference guide.

The guide now contains supplemental, rather than duplicate,
information relative to the "App Shortcuts" guide, available at
https://developer.android.com/guide/topics/ui/shortcuts.html

Test: make ds-docs -j8

Bug: 64401518
Change-Id: Iefa34299a9765eb5d853da71d9197a0edb346026