Do not wait for URI metadata to display text in Img+Text UI

Image plus text content preview UI can display shared text right away
without waiting for the shared URI metadata.
Headline can be derived from the target intent's mime type and refined
when the metadata is loaded.

Bug: 292157413
Test: manual testing with ShareTest app
Test: unit tests
Change-Id: Icbdc1cccc16643e7e49be6dbfe9499db0278d9ac
3 files changed