[DocsUI tests] Set withToolbarTitle mismatch description

Previously is the withToolbarTitle matcher failed, we would get
an error message like this, where the actual toolbar title was
not mentioned:

Expected: with toolbar title: is "Virtual SD card"
Got: com.google.android.material.appbar.MaterialToolbar{5683a01 V.E
...... ........
0,0-880,64 #7f09016d app:id/toolbar}


Override describeMismatch() to state the actual toolbar title
to get something like:

Expected: with toolbar title: is "Virtual SD card"
Got: unexpected toolbar title: "Brya"


Flag: EXEMPT this is not behind a flag
Bug: 438851396
Test: com.android.documentsui.FileCopyUiTest
Change-Id: I42cc758761c79e06118b39c5523e4ae4e9584565