Slight API renaming, better behavior.

Rename APIs to reflect that they're storage-related.  Also move
credential-storage APIs to be system API.

Return a null Context when device-encrypted storage isn't
supported.  This is the easiest way to keep legacy apps working when
upgrading from M to N.

Reduce strictness of path checking so we don't crash when working
with special packages like "android".

Bug: 22358539, 26104027
Change-Id: I38c24fc003488186210a6ae3b64270f86e1efe56