Add (hidden for now) purgeable bitmaps

BitmapFactory::Options now let you specify if the resulting bitmap can be
"purgeable". If so, then its decoded pixels may be purged when not actively
being drawn, freeing up that RAM. When such a bitmap is drawn, it will
automatically be re-decoded on demand. This is done by having the bitmap
keep a reference/copy of the encoded data.

Where it is a reference or a copy is controlled by the "shareable" flag in
Options. If this is true, the implementation *may* just reference the encode
data (e.g. a file descriptor) rathern than making a complete copy of it.

Currently, purgeable is not supported for generic inputstreams, but is
enabled for byte-array, file-descriptor, and assets, though for impl
reasons only file-descripts are currently enabled for "shareable", but that
may change in the future.