Explicitly disable flag for some clipboard tests

There are a couple tests that were testing behavior when the input
clipdata is 'theoretically' an image, but calling
contentResolver.getType on the URI fails. With the change to read the
mimetype from the clipdata, this check is obsolete (since we're testing
the case where the mimetype isn't 'image' separately, as a
'nonImageUri').

This change explicitly disables the "use_description_mimetype" flag for
those tests; once that flag is rolled out completely the tests can be
removed.

Bug: 371244749
Bug: 357197236
Test: atest ClipboardOverlayControllerTest (with the mimetype flag
turned on on the device)
Flag: EXEMPT adjusting flagged test behavior

Change-Id: Ib231f8b0965a004caa0496770ebc7068c0556786