Allow purging of asset bitmaps

It seems that neither CM9 nor CM10.x includes the actual framework-side
code to facilitate user-controllable bitmap purging. Odd, since the
CyanogenMod Settings app in both CM9/CM10.x includes the option.

Backported from original Gingerbread commit:
http://review.cyanogenmod.org/#/c/3908

Original description:

Allow the system to reclaim more free RAM when needed on 'low' memory devices
(e.g. Droid/Milestone) that are struggling with 2.3. This option enabled
efectively restores the froyo way of forcePurgeable BitmapFactory asset
handling.

Change-Id: Ia7efca91fb74dda39f4d176875ee8a66bb34509d