Remove activity icon bitmaps from system process.

When a TaskDescription is sent up to the system process, the Bitmap
contained in the mIcon member is immediately flushed to disk and the
name of the file replaces it in the TaskDescription. Thereby saving
mucho RAM for better uses.

Fixes bug 17527308.

Change-Id: Ifac63ea5d12ed091d1eb03e178b8b847a827f940