Load assets in place instead of deferring until draw.

Before this change, all framework assets would be decoded at drawing time
outside of zygote. This was forcing all apps to re-decode the assets and
zygote to keep an in-memory copy of each asset. This behavior is now
opt-in by setting the inPurgeable flag on BitmapFactory.Options.

Change-Id: Ic703f57adb26b2a701ecff0a653d35a93e26d47c