Dragging the best available activity from Recents (to Pinned).

I experimented with various apps that have multiple activities, and
found the approach that seem to work fine in most cases.

The general idea is to preserve the task's activity if it's launcheable,
or to drag the apps launch activity otherwise.

Bug: 20024603
Change-Id: Ieb64423874f55f64f089ce1db6cf205469756419