[DocsUI M3] Update a11y voice announcement for file selection

browsing mode:
* for unselected document:
  click -> open, long click -> select
* for selected document:
  click -> deselect, long click -> drag

picking mode:
* for unselected document:
  click -> select, long click -> select multiple
* for selected document:
  click -> deselect, long click -> no op

Check the attached bug for the demo.

Bug: 408898095
Test: m DocumentsUIGoogle && use TalkBack
Test: atest
DocumentsUIGoogleTests:com.android.documentsui.dirlist.AccessibilityTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ie4f729aae0b5b7706d0f7c243e691882ada15e9a