Detect content:// leaving apps without grants.

Developers regularly put content:// Uris into Intents, but they can
easily forget to add FLAG_GRANT_READ_URI_PERMISSION to actually
extend a permission grant to the receiving app.

Also fix NPE when path is missing.

Test: builds, boots, common actions work without triggering
Bug: 32447617, 31900890
Change-Id: Ic6054b1d73de50967cf7fe66abc293c60a41b97e