Introduce a new DocumentsDefaultM3Theme behind a flag

This theme will serve as the entry point for the material 3 uplift.
It also parents at the Theme.Material3.DynamicColors theme and applies
DynamicColors to the current activity when available.

Currently the theme is a copy paste from the existing theme with the
parent updated. Subsequent CLs will repoint the style references to
their corresponding M3 variants or remove them entirely if the
underlying theme is WAI.

Bug: 377771410
Test: m DocumentsUIGoogle && adb install DocumentsUIGoogle.apk
Flag: com.android.documentsui.flags.use_material3

Change-Id: Ib0ab7ed89f879304ebd475e316b5d1f16d3ee889