[Android] Set targetApiVersion to 25

We keep being compatible with versions all the way down to 16 for now, but this
will disable internal compatibility shims when running on 25.

android/build.gradle

     compileSdkVersion = 25
     buildToolsVersion = "25.0.3"
     minSdkVersion    = 16
-    targetSdkVersion = 23
+    targetSdkVersion = 25
 }
 
 // Force the version of the Android build tools we have chosen on all