Change API level into 21

Signed-off-by: Leo Ma <begeekmyfriend@gmail.com>

app/build.gradle

@@ -6,7 +6,7 @@ android {
 
     defaultConfig {
         applicationId "net.ossrs.yasea.demo"
-        minSdkVersion 16
+        minSdkVersion 21
         targetSdkVersion 22
         versionCode 1
         versionName "2.6"

library/build.gradle

@@ -5,7 +5,7 @@ android {
     buildToolsVersion "26.0.2"
 
     defaultConfig {
-        minSdkVersion 16
+        minSdkVersion 21
         targetSdkVersion 22
         versionCode 1
         versionName "2.6"