Migrate DocumentsUIGoogle for mainline module

Update targetSdk to 29(Q) for mainline module
- Define DocumentsUI as a static_library
- DocumentsUIGoogle import DocumentsUI and sign mainline key

Test: make DocumentsUI, DocumentsUIGoogle
      ensure both DocumentsUI & DocumentsUIGoogle can build pass
      DocumentsUIGoogle.apk will be built in
      /product/priv-app/DocumentsUIGoogle/
      atest DocumentsUITests
      atest DocumentsUIGoogleTests
Fix: 123607299

Change-Id: Ie76506d2e57b023772148c1268191430c05a82fb