chore: update minSdkVersion to get the project version or fallback to 21

Signed-off-by: Jose Bravo <ja.bravo.isidro@gmail.com>

wrappers/javascript/aries-askar-react-native/android/build.gradle

@@ -59,6 +59,10 @@ def getExt(name) {
   return rootProject.ext.get(name)
 }
 
+def getExtWithFallback(prop, fallback) {
+  return rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 def resolveBuildType() {
     def buildType = "debug"
     tasks.all({ task ->
@@ -81,7 +85,7 @@ android {
   }
 
   defaultConfig {
-    minSdkVersion 21 
+    minSdkVersion getExtWithFallback('minSdkVersion', '21') 
     targetSdkVersion getExt('targetSdkVersion')
 
     externalNativeBuild {