Allow us to specify android-prefix for non-bnc.lt link domains

[rex-iotum]

Currently android-prefix is purely for use with bnc.lt, since its validations severely limit what can be put there. (3-4 char alphanumeric paths only)

I want to be able to specify a custom link-domain while able to specify a custom android-prefix.
Without it there is no way to limit which paths can be deeplinked for Android using this plugin.

IE with this config of
<link-domain value="*.callbridge.com" />
<android-prefix value="/conf/call/" />
to generate the below.
<data android:host="*.callbridge.com" android:pathPrefix="/conf/call/" android:scheme="https"/>
Instead of throwing BRANCH SDK: Invalid "android-prefix" in <branch-config> in your config.xml. Docs https://goo.gl/GijGKP

I believe the code already support this, but I am being stopped by this validation !/^[/].[a-zA-Z0-9]{3,4}$/.test(preferences.androidPrefix)