Separate ImageDecoder JNI from actual work

Bug: 135133301
Test: CtsGraphicsTestCases ImageDecoderTest

Make the native ImageDecoder a class, with methods that do the bulk of
the work that was previously done in JNI. This will allow the
forthcoming NDK API (AImageDecoder) to share the same code for decoding.

Move enums into implementation file, as no other code needs them.

Make computeAllocationSize a public static method on hwui/Bitmap, for
use by ImageDecoder.

Change-Id: I4e4dae338a951761614aed42ca2cc157e3d526dd