Fix for share button on iPad

Activity view controller must be presented in a popup on iPad,
but we were presenting it modally. This started crashing on
iOS 8. Nice! So, let's fix it.

Amended to fix "popover dealloc'ed while popover still
visible" crash on iPad 2 iOS 7.1 simulator.

Bug: 71189
Change-Id: I6da0feff6b415b9ac6e23bf712b85baf3a14188e