[DocsUI, Search]: Compute a week ago when searching

The code computes a week ago in ms constants when SearchChipViewManager
is created, rather than when search is executed. Thus it is possible to
have SearchViewChipManager created one day, and search executed the
following day, leading to 1 day difference between what the user would
expect ot be one week ago, and what the code uses.

Bug: 431654697
Test: atest DocumentsUIUnitTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I019080f9294b225b2d9b29284e6cb33b04c302b5