Use a valid mime type in ChooserActivityTest cases

This doesn't affect any of our currently-enabled tests,
but as in b/208837849 it can apparently cause behavioral
changes in our currently-disabled logging tests; in order
to re-enable those tests for b/208803748, we'll need to
specify one of these behavior cases and should prefer to
match what happens when the mime type *is* valid. "Golden"
event sequences can be re-recorded for those tests after
this CL is merged, and that should be sufficient to
re-enable them.

More concerningly, an earlier attempt to re-enable some
ChooserActivityTest cases resulted in flakes when an
exception was thrown parsing the invalid mime types
(b/208898133). It's unclear why this wouldn't fail more
consistently, and perhaps that's why these tests were
disabled in the first place -- but if we have specific
intentions in this case, we should test them explicitly
instead of allowing them to interfere with test cases
that were simply trying to set up "typical" conditions.

Test: `atest ChooserActivityTest`
Bug: 209804664
Change-Id: I02854aac788d36165ad9e0eabd6f5e02e1d1204d