Upgrades min SDK from 16 to 21

Seems to be to required to run tests in Android on modern react native versions (at least since 2018)

closes #59

Author: redreceipt

android/build.gradle

@@ -21,7 +21,7 @@ android {
     compileSdkVersion safeExtGet('compileSdkVersion', 29)
     buildToolsVersion safeExtGet('buildToolsVersion', '29.0.2')
     defaultConfig {
-        minSdkVersion safeExtGet('minSdkVersion', 16)
+        minSdkVersion safeExtGet('minSdkVersion', 21)
         targetSdkVersion safeExtGet('targetSdkVersion', 29)
         versionCode 1
         versionName "1.0"