fix(Android): read minSdkVersion from root project (#314)

Author: keremoge

android/build.gradle

@@ -24,7 +24,7 @@ android {
     buildToolsVersion rootProject.hasProperty('buildToolsVersion') ? rootProject.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
 
     defaultConfig {
-        minSdkVersion 15
+        minSdkVersion safeExtGet('minSdkVersion', 15)
         targetSdkVersion rootProject.hasProperty('targetSdkVersion') ? rootProject.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
         versionCode 1
         versionName "1.0"