Support Dark/Light Theme for DocumentsUI (2/N)

1) Define Light Theme and ToolBarTheme to match M2 design
2) Adjust GridItem / ListItem background in Dark Theme
3) Tuning StatusBar and NavigationBar color fulfill design
4) Refactor legacy style definition for GridItem

Test: atest com.android.documentsui.dirlist.ThemeUiTest
      atest com.android.documentsui.dirlist.DarkThemeUiTest
      manually swich to night mode

Bug: 111862978
Bug: 114369969
Bug: 114805149
Change-Id: I3029a4f25640f83cfd5986f965923656fd01958d