Make PendingIntents in screenshots immutable

S and later will require PendingIntents to explicity specify whether
they are mutable or immutable.

Test: manual, just a flag change (tested that normal screenshot
flow works, verified that the log nags for unspecified mutability
disappeared)

Change-Id: I79b78f80c2d6af07a3826b6f79d0d86a5fc2f756