[DocsUI, Search]: Code cleanup.

Puts all debug logs behind debug. Uses Kotlin standards for instance
variable names. Fixes warnings about null duration.

Bug: 415881678
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: Ic1e4f24ae4dbde6eea00514f87d756bfeff51497