Add documentation for ImageDecoder and its Source

Bug: 76448408
Bug: 76462125
Test: Iec21bad971dc9cffc807a412bb6329757c499baa

- Document DecodeException.getSource
- Add thread annotations
- slice() ByteBuffer inside createImageDecoder, so it can be reused
- Make ResourceSource thread safe by locking around mResDensity
- Specify that OnHeaderDecodedListener is necessary for changing default
  settings

Change-Id: I3b55d3ba1b0a2276938cb521449bceb7aa9f96e2