Only run clipboard shared transition for images

There's no available shared transition for text, so trying to run a
shared transition looks bad. Switch to only running the shared
transition for images, otherwise regular dismiss + share intent.

Bug: 369132381
Fix: 369132381
Test: manual (invoke share for text content)
Flag: com.android.systemui.clipboard_shared_transitions
Change-Id: Iecaa3d86a512cc85d7af845403f352381dd14b0a