Unhide BitmapRegionDecoder.

1. Rename LargeBitmap to BitmapRegionDecoder
2. Move the instantiations of BitmapRegionDecoder out of BitmapFactory.
3. Remove the use of MemoryFile in BitmapRegionDecoder, since MemoryFile's API had been modified in master. Otherwise, the change will break the master build.
4. Move AssetStreamAdaptor, AutoFDSeek and nullObjectReturn to Utils.h because BitmapFactory.cpp and BitmapRegionDecoder.cpp both need to use these utility functions.

Most of the modifications, except for (2) and (3), were reviewed in https://android-git.corp.google.com/g/#change,64716 .
However, that change broke the master build due to (3) and was reverted eventually.
So, instead of withdrawing this change and waiting for that change to be checked in again, I merge the two changes into one.

Change-Id: I2202c0fbbbd6d6676bbd9637e690023ea4099c40