Change android minimum SDK version to 14

Summary:
The library(yoga) has no dependency on android framework. As a matter of
fact this version can change to a lower number which will make yoga
support more old systems.

According to I/O 17, support library has change its minimum SDK version
 to 14, so I think 14 is a reasonable min support version for yoga as well.

See
https://developer.android.com/topic/libraries/support-library/revisions.html
Closes #573

Differential Revision: D5264385

Pulled By: emilsjolander

fbshipit-source-id: cb813a3b643a3f4e2034c6c0ab8b488f94d6a7d0

Author: sospartan

build.gradle

@@ -22,7 +22,7 @@ allprojects {
 }
 
 ext {
-    minSdkVersion = 15
+    minSdkVersion = 14
     targetSdkVersion = 25
     compileSdkVersion = 25
     buildToolsVersion = '25.0.2'