allow untrusted_app to write to MMS files

Commit 3fbc536d allowed untrusted
app to read radio data files passed via binder, but didn't allow
write access. Write access is needed when sending MMS messages.

Steps to reproduce:
1) have some photos on the device
2) Launch messaging app
3) Attach a MMS (Picture, capture video, capture picture, audio recording etc..)
4) Send

EXPECTED RESULTS:
No crash

OBSERVED RESULTS:
- Messaging crashes on sending MMS
- messages are stuck in sending state

Additional details:
  05-05 10:14:01.196  2457  2457 W Binder_3: type=1400 audit(0.0:20): avc:  denied  { write } for  path="/data/data/com.android.providers.telephony/app_parts/PART_1399310041183_temp.jpg" dev="mmcblk0p23" ino=604417 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:radio_data_file:s0 tclass=file
  05-05 10:14:01.202 27809 28219 E JavaBinder: !!! FAILED BINDER TRANSACTION !!!
  05-05 10:14:01.203 27809 28219 E PduPersister: Failed to open Input/Output stream.
  05-05 10:14:01.203 27809 28219 E PduPersister: java.io.FileNotFoundException: Failed opening content provider: content://mms/part/4
  05-05 10:14:01.203 27809 28219 E PduPersister:        at android.content.ContentResolver.openAssetFileDescriptor(ContentResolver.java:966)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at android.content.ContentResolver.openOutputStream(ContentResolver.java:674)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at android.content.ContentResolver.openOutputStream(ContentResolver.java:650)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.google.android.mms.pdu.PduPersister.persistData(PduPersister.java:837)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.google.android.mms.pdu.PduPersister.persistPart(PduPersister.java:761)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.google.android.mms.pdu.PduPersister.persist(PduPersister.java:1398)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.android.mms.data.WorkingMessage.createDraftMmsMessage(WorkingMessage.java:1577)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.android.mms.data.WorkingMessage.sendMmsWorker(WorkingMessage.java:1431)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.android.mms.data.WorkingMessage.access$700(WorkingMessage.java:82)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at com.android.mms.data.WorkingMessage$2.run(WorkingMessage.java:1228)
  05-05 10:14:01.203 27809 28219 E PduPersister:        at java.lang.Thread.run(Thread.java:818)
  05-05 10:14:01.221 27809 28219 E AndroidRuntime: FATAL EXCEPTION: WorkingMessage.send MMS
  05-05 10:14:01.221 27809 28219 E AndroidRuntime: Process: com.android.mms, PID: 27809
  05-05 10:14:01.221 27809 28219 E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.net.Uri.getLastPathSegment()' on a null object reference
  05-05 10:14:01.221 27809 28219 E AndroidRuntime:      at android.content.ContentUris.parseId(ContentUris.java:85)
  05-05 10:14:01.221 27809 28219 E AndroidRuntime:      at com.android.mms.model.SlideshowModel.finalResize(SlideshowModel.java:691)
  05-05 10:14:01.221 27809 28219 E AndroidRuntime:      at com.android.mms.data.WorkingMessage.sendMmsWorker(WorkingMessage.java:1448)
  05-05 10:14:01.221 27809 28219 E AndroidRuntime:      at com.android.mms.data.WorkingMessage.access$700(WorkingMessage.java:82)
  05-05 10:14:01.221 27809 28219 E AndroidRuntime:      at com.android.mms.data.WorkingMessage$2.run(WorkingMessage.java:1228)
  05-05 10:14:01.221 27809 28219 E AndroidRuntime:      at java.lang.Thread.run(Thread.java:818)
  05-05 10:14:01.222   659  5253 W ActivityManager:   Force finishing activity com.android.mms/.ui.ComposeMessageActivity

Bug: 14562421
Change-Id: Iba6914eeec4bf0c8c04ee83584327a4824c0a9a9