update to newer Android SDK version

android/olm-sdk/build.gradle

@@ -3,12 +3,11 @@ import org.apache.tools.ant.taskdefs.condition.Os
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 27
-    buildToolsVersion '27.0.3'
+    compileSdkVersion 28
 
     defaultConfig {
         minSdkVersion 11
-        targetSdkVersion 21
+        targetSdkVersion 28
         versionCode 311
         versionName "3.1.1"
         version "3.1.1"
@@ -120,7 +119,7 @@ def gitRevisionDate() {
 dependencies {
     testImplementation 'junit:junit:4.12'
     androidTestImplementation 'junit:junit:4.12'
-    androidTestImplementation 'com.android.support:support-annotations:27.1.1'
+    androidTestImplementation 'com.android.support:support-annotations:28.0.0'
     androidTestImplementation 'com.android.support.test:runner:1.0.2'
     androidTestImplementation 'com.android.support.test:rules:1.0.2'
 }

android/olm-sdk/src/main/jni/Application.mk

 APP_PLATFORM := android-16
 APP_ABI := arm64-v8a armeabi-v7a x86_64 x86
-APP_STL := gnustl_static
+APP_STL := c++_static
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