Update test to only work with sMutableColumns.

Apps can only modify columns in the set of sMutableColumns, so this
change updates the test to use the BOOKMARK column, since neither
DESCRIPTION nor IS_PRIVATE are mutable.

Bug: 155352327
Test: atest CtsProviderTestCases:android.provider.cts.media.MediaStore_FilesTest
Change-Id: Ibf49b363a85813cce30f74d9d01e48620530f87f
(cherry picked from commit 1308c53792ebb3546f4b10181ce976fb5010020d)