[DocsUI Peek] Conditionally instantiate PeekViewManager in tests

The PeekViewManager is optimized out of DocumentsUI when Peek is
disabled. In tests, the TestPeekViewManager should only be
instantiated when Peek is enabled.

Bug: 423014550
Bug: 423179676
Bug: 423233932
Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.use_peek_preview_ro
Test: DocumentsUIGoogleTests com.android.documentsui.files.ActionHandlerTest
passes when Peek is disabled

Change-Id: Icf5fa50ab12a3868f278b65c14e5be29843a56d5