LatinIME: Remove Android.mk usage

Rename the jni makefile that's used for gradle builds and remove
the empty makefile that was used as a guard.

Change-Id: Ibcd1b29f1297d3f4b2b01682a100eba8d68a53b8

build.gradle

@@ -76,7 +76,7 @@ android {
 
     externalNativeBuild {
         ndkBuild {
-            path 'native/jni/Android.mk'
+            path 'native/jni/AndroidNdk.mk'
         }
     }
     androidResources {