Fix incorrect syntax for class names in JNI.

Causes a crash on Android L/ART.
Android: Prevent editing of files opened from a memory buffer.

Memory buffers are used for implementing content:// URLs, which are
(in most cases) readonly. If we ever encounter a read/write content://
URL in the future, we could consider supporting saving to it.

(An example of a content:// URI is an email attachment, where IPC is used
to transfer the file from the email client, rather than relying on a local
file).
Don't crash if a document fails to open on iOS.

Handle exceptions from fz_open_document().