Android MMS api allows arbitrary file access as the radio user

Bug 8214355

Only allow files in mms's app_parts directory to be opened.
Tested with the code found in the bug, before and after this
change.

(cherry picked from commit 8168ff8cebcd1b51ef9d97f2a50f9c91ee8aa562)

Change-Id: Ieab4cf20f6191aba5c2118d454dcf69f5e268308
1 file changed