Update ReactAndroid build script to support gradle 2.3.0

Summary:
We updated to Gradle 2.3.0 and our app's build failed. Our app doesn't provide "repositoryUrl" which is intended to be an optional gradle property. However, Gradle 2.3.0 blows up on findProperty('repositoryUrl') when "repositoryUrl" isn't provided:

````
* What went wrong:
A problem occurred configuring project ':ContextMenuAndroid'.
> Could not resolve all dependencies for configuration ':ContextMenuAndroid:_debugPublish'.
   > A problem occurred configuring project ':ReactAndroid'.
      > Could not get unknown property 'repositoryUrl' for project ':ReactAndroid' of type org.gradle.api.Project.
````

To fix this, we now use "project.hasProperty('repositoryUrl')" to safely detect the presence of the optional "repositoryUrl" property.

Since I cannot check it with your build environment, I've created a small demo to show that "project.hasProperty" properly detects the presence of the gradle property "repositoryUrl". I edited "getRepositoryUrl" to throw an exception if "repositoryUrl" is set:

````
def getRepositoryUrl() {
    if (project.hasProperty('repositoryUrl')) throw new GradleException(property('repositoryUrl'))

    return project.hasProperty('repositoryUrl') ? property('repositoryUrl') : 'https://oss.sonatype.org/service/local/staging/deploy/maven2/'
}
````
Then I ran gradle with "repositoryUrl" set like this (passing the property):
````
./gradlew -PrepositoryUrl=blah assembleDebug
````

As expected, it detected that "repositoryUrl" was set and threw an exception:
````
* What went wrong:
A problem occurred configuring project ':ContextMenuAndroid'.
> Could not resolve all dependencies for configuration ':ContextMenuAndroid:_debugPublish'.
   > A problem occurred configuring project ':ReactAndroid'.
      > blah
````

The same issue has been reported before - #14811, #14810

Minor changes in the Android build script
Closes #18075

Differential Revision: D7077788

Pulled By: hramos

fbshipit-source-id: ecfbab29d0632e7eecb3c6a247df39bc7616653e

Author: Sasha Nikiforov

Diff for: ReactAndroid/release.gradle

@@ -12,15 +12,15 @@ def isReleaseBuild() {
 }
 
 def getRepositoryUrl() {
-    return hasProperty('repositoryUrl') ? property('repositoryUrl') : 'https://oss.sonatype.org/service/local/staging/deploy/maven2/'
+    return project.hasProperty('repositoryUrl') ? property('repositoryUrl') : 'https://oss.sonatype.org/service/local/staging/deploy/maven2/'
 }
 
 def getRepositoryUsername() {
-    return hasProperty('repositoryUsername') ? property('repositoryUsername') : ''
+    return project.hasProperty('repositoryUsername') ? property('repositoryUsername') : ''
 }
 
 def getRepositoryPassword() {
-    return hasProperty('repositoryPassword') ? property('repositoryPassword') : ''
+    return project.hasProperty('repositoryPassword') ? property('repositoryPassword') : ''
 }
 
 def configureReactNativePom(def pom) {