Sharesheet - App stacking.

When an app offers more than one activity to handle Send intents,
"stack" these into a single target in the All Apps list.

Tapping this target gives a disambiguation dialog to allow the
user to choose the specific action they want.

Long-pressing this app still brings up the app pinning dialog, but
app pinning allows you to pin any of the actions.

Test: atest ChooserActivityTest
Test: manual - press/longpress targets in A-Z list
Change-Id: Ida56344c3451da80d7cbe61bac8ca2117497e59a