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
fix NoSuchElementException error with CPU ProcessStats

Credit to Eugene373

p2: update commit message
p3: cleaned up tabs
p4: also added aosp changes from this commit - https://github.com/android/platform_frameworks_base/commit/ca894b396b93344ac255746e12730cb269d4d856

Signed-off-by: Jonathan Steadman <xxwhitehawk@gmail.com>
Change-Id: I3c71dbd5fb8e42811c47d6431003101943cb388a