[DocsUI M3] Add a DocsUI flag for adopting material 3

The flag will be used in both Java code and XML files, resource
flag needs to be read only.

Bug: 373720657
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_material3
Change-Id: I5b8e7734a4826137927d13fc0b29f874642b3ed5