[DocsUI, Search]: Minor code cleanup.

Use recommended kotlin standards for variable naming. Use a bit shorter
way of handling menu clicks. Add chip spies to increase code coverage.

Bug: 415881678
Test: m DocumentsUIGoogle
Test: m SearchOptionsControllerTest
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I2d76040aa5a36726c0cd51c905dc3b36ef333960