android: don't use minSdkVersion set by app (react-native-webrtc#1625)

Author: davidliu

android/build.gradle

@@ -14,7 +14,7 @@ android {
     buildToolsVersion safeExtGet('buildToolsVersion', "23.0.1")
 
     defaultConfig {
-        minSdkVersion safeExtGet('minSdkVersion', 24)
+        minSdkVersion 24
         targetSdkVersion safeExtGet('targetSdkVersion', 24)
         versionCode 1
         versionName "1.0"