Fix Gallery2 path traversal bug

Fix a path traversal bug that allowed a malicious launcher app to access files in the private space of the Gallery2 app by injecting a bad path.
Fix: Check URI before passing the file out from SharedImageProvider and raise exception if it's not pointing to the relevant folder.
Bug: 198174170
Test: manual by following the reporters instructions

Change-Id: I74dce6c4794b3eafb922bb9a2f2516677cc5f4d8