Solve build target error while building Android with Buck.
Solve build target error while building Android with Buck.

Summary:
SoLoader library targets Android API 21 but main manifest targets 19. This will block buck android build.
Closes https://github.com/facebook/yoga/pull/437

Reviewed By: astreet

Differential Revision: D4611364

Pulled By: emilsjolander

fbshipit-source-id: 113bcfce751e98b8a42e46e0101b6a4b1327b9c3

facebook/yoga,yihuang/css-layout,facebook/css-layout,yihuang/css-layout,facebook/css-layout,yihuang/css-layout,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,yihuang/css-layout,yihuang/css-layout,facebook/yoga,yihuang/css-layout,yihuang/css-layout,facebook/yoga,yihuang/css-layout,yihuang/css-layout,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,yihuang/css-layout,facebook/yoga