[DocsUI Peek] Fix background selection test

Follows up ag/33284525, the selection hotspot is not used properly.

Bug: 411295692
Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.use_peek_preview_ro
Test: PeekUiTest#testFileCantBeSelectedDuringFilePreview
Change-Id: Ifa43f2c783cc21c4f1cfa95110ea0ad6e6a6853e