OPP: Restrict file based URI access to external storage

* Allow only external storage paths in file based URI in
  BluetoothOppSendFileInfo when the file send request comes from an
  external source
* Fix a potential NPE when using Uri.getPath()

Bug: 35310991
Test: Make, test various cases of Bluetooth file share
Change-Id: I8ff00d63d3c880667302f8d7ff8eaa0c0b533921
(cherry picked from commit 3edd7f0a)