Update docs on splitting APKs (#496)

Gradle v3.1 complains about conflicting configuration as below

<pre>
Conflicting configuration : 'armeabi-v7a,x86' in ndk abiFilters cannot be present when splits abi filters are set : x86,armeabi-v7a
</pre>

Author: alvinthen

docs/signed-apk-android.md

@@ -112,6 +112,9 @@ By default, the generated APK has the native code for both x86 and ARMv7a CPU ar
 You can create an APK for each CPU by changing the following line in android/app/build.gradle:
 
 ```diff
+- ndk {
+-   abiFilters "armeabi-v7a", "x86"
+- }
 - def enableSeparateBuildPerCPUArchitecture = false
 + def enableSeparateBuildPerCPUArchitecture = true
 ```