Implement document trash flow APIs

Add support for document trash flow operations across
browsable reliable volumes, including external_primary
and SD cards. Also splitting FlakeStorageManagerTests and
ExternalStorageProviderTests for clarity.

Bug: 409260889
Test: atest ExternalStorageProviderTest
Flag: android.provider.enable_documents_trash_api
Change-Id: Ibab2accb985143144ed7562fff072e6402704ffb