Support GSTREAMER_ROOT_ANDROID env var

If GSTREAMER_ROOT_ANDROID is set, try using
that for the location of the GStreamer Android tree,
but still allow it to be overridden in local
gradle properties

playback/player/android/app/build.gradle

@@ -19,7 +19,10 @@ android {
                 if (project.hasProperty('gstAndroidRoot'))
                     gstRoot = project.gstAndroidRoot
                 else
-                    throw new GradleException('"gstAndroidRoot" must be defined in your gradle.properties to the top level directory of the unpacked universal GStreamer Android binaries')
+                    gstRoot = System.env.GSTREAMER_ROOT_ANDROID
+
+                if (gstRoot == null)
+                    throw new GradleException('GSTREAMER_ROOT_ANDROID must be set, or "gstAndroidRoot" must be defined in your gradle.properties in the top level directory of the unpacked universal GStreamer Android binaries')
 
                 arguments "NDK_APPLICATION_MK=src/main/jni/Application.mk", "GSTREAMER_JAVA_SRC_DIR=src/main/java", "GSTREAMER_ROOT_ANDROID=$gstRoot", "GSTREAMER_ASSETS_DIR=src/main/assets"
 

playback/player/android/gradle.properties

@@ -16,4 +16,8 @@
 # This option should only be used with decoupled projects. More details, visit
 # http://www.gradle.org/docs/current/userguide/multi_project_builds.html#sec:decoupled_projects
 # org.gradle.parallel=true
-gstAndroidRoot=/home/slomo/Projects/gstreamer/head/gst-examples/playback/player/gst-android
+
+# gstAndroidRoot can be set to point to the unpacked GStreamer android top-level directory
+# containing each architecture in subdirectories, or else set the GSTREAMER_ROOT_ANDROID
+# environment variable to that location
+# gstAndroidRoot=/path/to/gstreamer/android