[TIMOB-25206] Fix TiApplication activity type

[garymathews]

• Fix TiBaseActivity return type in TiApplication

TEST CASE

var win = Ti.UI.createWindow({backgroundColor: 'gray'});

win.addEventListener('open', function(e) {
    alert('is window activity current activity? ' + (e.source.getActivity() === Ti.Android.currentActivity));
});

win.open();

TIMOB-25206
TIMOB-20037

[mukherjee2]

Node Version: 6.10.3
NPM Version: 3.10.10
Mac OS: 10.12.4
Appc CLI: 6.2.3
Appc CLI NPM: 4.2.9
Titanium SDK version: 6.2.0.v20170830085903
Appcelerator Studio, build: 4.9.1.201707200100
Xcode 8.3.2
Android OS 7.1.2
Android phones: Pixel

Passed FR. Alert showed window activity as current activity as "true". I tried it on the affected version 6.1.2 where I expected false, and indeed I saw a false. FR passed. CR passed by Yordan in PR-9372