Support external android sdk version

[oncecrossover]

Please update build.gradle to Support external android sdk version

apply plugin: 'com.android.library'

def safeExtGet(prop, fallback) {
    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
}

android {
    compileSdkVersion safeExtGet('compileSdkVersion', 28)
    buildToolsVersion safeExtGet('buildToolsVersion', '28.0.3')

    defaultConfig {
        minSdkVersion safeExtGet('minSdkVersion', 16)
        targetSdkVersion safeExtGet('targetSdkVersion', 28)
        versionCode 1
        versionName "1.0"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:23.0.1'
    compile 'com.facebook.react:react-native:+'
    compile 'com.google.zxing:core:3.2.0'
}