DocsUI: Fix Search tests related to show_docked_search

Fix the config value for show_docked_search when material3 flag is
enabled. It requires to use the getRes() to be able to overload only
when the material3 is enabled via config, in addition to enabled via
flag.

Test: com.android.documentsui.SearchViewUiTest
Bug: 425591387
Flag: com.android.documentsui.flags.use_material3
Change-Id: I012d87ce9b415ffb457f94bd20e7d32ed0a4a8ff