Reduce minSdkVersion to 18 as this is the actual lower limit to support the package as is (#21)

18 is where GLES3 support was added.

Author: ryantrem

Apps/Playground/android/build.gradle

@@ -3,7 +3,7 @@
 buildscript {
     ext {
         buildToolsVersion = "28.0.3"
-        minSdkVersion = 24
+        minSdkVersion = 18
         compileSdkVersion = 28
         targetSdkVersion = 28
         ndkVersion = "21.0.6113669"