Icon: a clean, parcelable place for images.

Binder APIs which wish to consume Bitmaps *and* drawable
resources can now do so by using Icon, a kind of union type
that accommodates each of these. Icon also accepts byte
arrays holding compressed Bitmaps (PNG, JPEG, etc), which
saves clients the additional memory cost of decoding and
sending full uncompressed bitmaps through Binder interfaces.
Receiving clients can call loadDrawable{,Async} and then
getDrawable to start immediately using the image in an
ImageView or other Drawable-hosting container.

Bug: 19609468
Change-Id: Ic1343711c2ac0b15876b46f0b6008b0108a49470