Android: Allow NDK location to be overridden

Source/Android/app/build.gradle

@@ -161,25 +161,33 @@ String getGitPath() {
 }
 
 String getNdkPath() {
-    try {
-        def stdout = new ByteArrayOutputStream()
-
-        exec {
-            // ndk-build.cmd is a file unique to the root directory of android-ndk-r10e.
-            commandLine 'locate', 'ndk-build.cmd'
-            standardOutput = stdout
-        }
-
-        def ndkCmdPath = stdout.toString()
-        def ndkPath = ndkCmdPath.substring(0, ndkCmdPath.lastIndexOf('/'))
-
-        project.logger.quiet("Gradle: Found Android NDK:" + ndkPath)
-
-        return ndkPath
-    } catch (ignored) {
-        project.logger.error("Gradle error: Couldn't find NDK.")
-        return null;
+    def propsFile = rootProject.file("build.properties")
+    def ndkPath = null
+    if (propsFile.canRead()) {
+        def buildProperties = new Properties()
+        buildProperties.load(new FileInputStream(propsFile))
+        ndkPath = buildProperties.ndkPath
     }
+    if (ndkPath == null) {
+        try {
+            def stdout = new ByteArrayOutputStream()
+
+            exec {
+                // ndk-build.cmd is a file unique to the root directory of android-ndk-r10e.
+                commandLine 'locate', 'ndk-build.cmd'
+                standardOutput = stdout
+            }
+
+            def ndkCmdPath = stdout.toString()
+            ndkPath = ndkCmdPath.substring(0, ndkCmdPath.lastIndexOf('/'))
+        } catch (ignored) {
+            project.logger.error("Gradle error: Couldn't find NDK.")
+        }
+    }
+    if (ndkPath != null) {
+        project.logger.quiet("Gradle: Found Android NDK: " + ndkPath)
+    }
+    return ndkPath
 }
 
 String getAbi() {