Convert warning about keystore into warning block in SignedAPKAndroid.md

Summary:
The current docs page [Generating a Signed APK](https://facebook.github.io/react-native/docs/signed-apk-android.html) contains a note about keeping your generated keystore safe, but this isn't well highlighted.

This commit highlights the notice in a warning blockquote to ensure people following the guide don't miss the importance of keeping the keystore safe as shown below:

<img width="666" alt="screen shot 2016-06-06 at 10 01 25" src="https://cloud.githubusercontent.com/assets/1863808/15817245/10e4e6d2-2bcf-11e6-9fe8-defc8a6ec93c.png">
Closes https://github.com/facebook/react-native/pull/7948

Differential Revision: D3393005

fbshipit-source-id: e9c2666a79134eccc1b1868fe850ee896e63266e