[DocsUI, Search]: Fix enable flag for SearchV2.

SearchV2 should be only enabled on those devices on which  DocumentsUI
RRO is enabled.

Bug: 427555916
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I1d9c63e52a3593610ebf5873faad32f18ad05030