[DocsUI, Search]: Prepare search for tracing.

Adds traces to the method that provide heavy lifting for directory
loading and searching. This is done for both v1 (existing) version of
directory handling and v2, the new implementation. Uses begin and end
 session methods with documentsui.searchvN.<Class>#<method> tag, where
vN is either v1 or v2.

Bug: 417078119
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I57bb05d12c461a0c0b7ffa6d0bca893062b15349