Allow purging of asset bitmaps

By chance I discovered that neither CM9 nor CM10.x includes the actual
framework-side code to enable 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: I226db6ce3a4f6e4d959a83df77cc531016cd9f87