Adding ofxAndroidLaunchBrowser(String url) to ofxAndroidUtils

This wasn't discussed as an issue... Sorry. I needed it, so it got implemented, and now I'm submitting it.

Similar to ofLaunchBrowser(string url) in ofUtils and ofxiPhoneLaunchBrowser(string url) in ofxiPhoneExtras.

Tested on an Android 2.3.4 device with SDK r21, NDK r8c and API 17.

I have a very simple example that I used for testing, that I can also submit with this PR.