[DocsUI]: Use isGreaterThan to get more information.

We have intermittent failure of test that check the height of various
dialogs. To get more information, this CL switches them to using
isGreaterThan matcher, which reports by how much we missed expectations.
isGreaterThan generates messages such as
 expected to be greater than: XXX
    but was                 : YY

Bug: 433379370
Test: atest DocumentsUIGoogleTests
Flag: EXEMPT testing code
Change-Id: Ide22064bcf06a853835e654fa48558168219210c