Also adjust to dynamic permissions after bumping target-SDK.

There still is some confusion about the concept of "external storage" in
the code. LocalDocuments already is "external storage" - clean that up a
little and use AppCompat function instead of using a legacy class for
ExternalDocuments provider.
Doesn't help for broken ROMs though, that would need guessing pathname
for a mounted SD (in addition to separate storage partition of builtin
storage).

Also c6e8c96d incorrectly changed the
conditional around, making the whole ExternalDocumentsProvider useless/a
copy of the Local one (i.e. the primary, first returned by the system).
Real fix for tdf#99539 likely was 66be4fee

Change-Id: I88ca7742c0f2e89d63c338c8852ad88be0a46e4b
Reviewed-on: https://gerrit.libreoffice.org/45572Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>