Read premultiplied status from the SkBitmap.

Replace the Java variable with mRequestPremultiplied, to better
reflect what it represents. In both native and Java, the SkBitmap
is used as the decision maker of whether a Bitmap is premultiplied.
When changing other settings, mRequestedPremultiplied is used to
determine whether it should be premultiplied (if the new config/
hasAlpha-ness etc supports it).

ChooseFromColorProc now reads both the colortype (instead of the
deprecated Config) and alphatype on the SkBitmap. Same with
ChooseToColorProc. In the process, this caught a bug, where the
wrong procs were being used for Index8.

Replace instances of SkBitmap::Config with SkColorType where I
was already changing code.

Use the new versions of setConfig/allocPixels that take an SkImageInfo
as a parameter.

Document isPremultiplied's return value for ALPHA_8.

BUG:13618134
Change-Id: I91fc1f1e46e9294364b1af0ab4bdb37c68d7058e