[TIMOB-6038] Expose tiapp.xml Props to App Properties

[dawsontoth]

Mix in the values from the tiapp.xml in to the App properties in addition to System properties. This is redundant, I know, but it touches the least amount of code.

[dawsontoth]

This gives API parity, but with a couple complications:

• tiapp.xml properties would overwrite existing properties.
• removing a property from tiapp.xml wouldn't remove the last set value in shared preferences.

[rusticphilosopher]

Noting comment left on Jira ticket for visibility:

"Reference info regarding Android implementation: I don't know why we even store the tiapp.xml properties inside the "systemProperties" map on Android. We should totally remove the systemProperties array and work solely with the appProperties map since all the documentation works off this assumption. If anyone has an objection to this, please comment.

Ideally, the generated App.java file would ONLY insert properties into a single properties map. For the time being, this pending fix will be accepted but we should implement a real fix at the platform level to remove the duplicate properties map."

[rusticphilosopher]

Code reviewed and functional test passed on Rhino and V8. Accepted