Replace setAlphaFlags with setUnpremultipliedRequired

Bug: 135133301
Test: I48e49ee08ab1954eddf62ecae87942aeb128c10d

As described in I3381582e27894e1072db9b8635f3762b801f5d69, this is a
more sensible API.

In addition, remove unused methods on ImageDecoder.

Lastly, update AImageDecoder methods in the map to document which API
level they were introduced in.

Change-Id: I1aff544e8d6932b9ed0931a00da66a0aba6cd536