ADT/Layoutlib: New API to let the caller instantiate the bitmap.

ADT/Layoutlib: New API to let the caller instantiate the bitmap.

This allows us to use a bitmap more compatible with SWT.

In ADT's case, because the bitmap needs to be converted to SWT
before being displayed, we create a BufferedImage using a byte[]
instead of a int[] so that we can simply do an array copy.

Also, we reuse the generated BufferedImage unless the size changed,
which lets us see less GC during animation playback.

Change-Id: I0062a4f4442ff6469cf0ad4f501c1fbe8c719400