Add FLAG_CANCEL_CURRENT to screenshot share intent

The share intent uses clipData, which isn't used to match intents.
Thus, subsequent screenshots didn't update the share intent, so
trying to share a second screenshot would end up sharing the first
one. Adding FLAG_CANCEL_CURRENT fixes this problem.

Bug: 159751741
Fix: 159751741
Test: manual
Change-Id: I5eb63f075c3156e5061ee9f63ba17afa6b7abbbd