[BUG] No equivalent to MotionEvent's getActionButton in NDK

#### Description

Hi, I'm trying to improve support for mouse input in my NDK-based apps by handling ACTION_BUTTON_PRESS & ACTION_BUTTON_RELEASE events. I found their NDK definitions in input.h but there isn't a corresponding AMotionEvent_getActionButton function to actually get the button that was pushed/released. Is this a bug or is there an alternate way of doing this?

#### Environment Details

* NDK Version: 24.0.7956693-beta2
* Build system: Custom
* Host OS: Gentoo Linux

Start with input.h and the existing AMotionEvent accessors, comparing the ACTION_BUTTON_PRESS and ACTION_BUTTON_RELEASE definitions with the available NDK API. Check the comment thread for any proposed alternate access path; done means the missing button value has a supported NDK access method or a documented alternative.

Written by the indexing model from the issue text.