[DocsUI, Search]: Fix testSearchV2LastUsedChipCopiedToFileTypeDropdown

Apparently, using getLast() is not yet available. Switching to
explicitly getting last element by index.

Bug: 430452915
Test: atest DocumentsUIGoogleTest
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I0460f408e18a3611513ff2cf94a83aa1a7163b61