make survey prompt optional via metadata flag

[grendell]

Currently in the Mixpanel SDK for iOS, there is no prompt before the user is shown a survey. We would like to replicate this on Android. The intention of this change is to allow this to be configurable via a metadata tag in the app manifest, falling back to current behavior if this tag is not present.

Example usage:

    <meta-data
        android:name="com.mixpanel.android.MPConfig.ShouldPromptForSurvey"
        android:value="false"/>

If there is a more appropriate place for this flag, please let me know.

[grendell]

Also, if there are any instructions for how to compile mixpanel-android locally, I have run into major issues both attempting to export an AAR and creating a submodule in our app, using Android Studio 1.5 and gradle 1.5.0. The AAR appears to be incomplete, while compiling the submodule leads to OOMs which I can't seem to solve with studio.vmoptions.

[grendell]

Updated based on feedback from #283.