[DocsUI, Search]: Remove level 15 API

CharSequence#isEmpty is API level 15. DocumentsUI must target API
level 11. Adjusting.

Bug: 415881678
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I6d1c25e2ad7d2619632db657717275ea58f5012d