Better setup for android build version.

[hyochan]

React Native now has its project set up with default build version of androids sdk. Set android build version to that of rootProject’s to prevent from build failing cause of version mismatch.

[maicki]

@thorbenprimke Could you please take a look at this PR? Thanks!

[hyochan]

@maicki @thorbenprimke I just got notified and I updated code safer.

[maicki]

@dooboolab Hey - I would like to go forward with this, just some small improvements before landing. Can you extract setting the version via a function like:

def safeExtGet(prop, fallback) {
     rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
 }

and set each value like:

// ...
compileSdkVersion safeExtGet('compileSdkVersion', DEFAULT_COMPILE_SDK_VERSION)
// ...

[maicki]

LGTM let's get it in and approve on top

[hyochan]

@maicki Someone already gave PR on this one. Thanks for the merge.