Add SkAndroidCodec::getPixels

Add SkAndroidCodec::getPixels

This is a synonym for the version of getAndroidPixels that accepts
only three parameters (i.e. no AndroidOptions). It is very similar
to SkCodec::getPixels, so I think the motivation for naming the
version with options differently does not apply here.

Add comments to the header describing defaults.

Update the test to use a template, and delete a lot of redundant code.

Rename a variable to stop shadowing another variable.

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1647153002

Review URL: https://codereview.chromium.org/1647153002