Fix for obscure bug on WP8

There is a bug on WP8 (Lumia 620, Lumia Black) with the Obscure event.
The bug occure sometimes when resume from the Lock screen and cause the
app to pause on a black screen.
It never happens when the phone is attacked to the debugger so this
required some sort of on-screen logging to figure out.

case #1
ApplicationIdleDetectionMode=Enabled (default)
App is suspended either by timeout or power button.

Obscured event triggered. IsLocked=true.
It's safe to ignore the Obscure event since the OS deactivates the app
anyway. IsActive is set to false by OnDeactivate.
When the user unlock the phone Unobscure event triggered but *sometimes*
another Obscured event
arrive after that, causing to app to remain in IsActive=false state.
That's why we need to ignore the event.

case #2
ApplicationIdleDetectionMode=Disabled
App is suspended either by timeout or power button.

Obscured event triggered. IsLocked=true.
We set IsActive=false on Obscure event as before.
When the user unlock the phone Unobscure event triggered and we set
IsActive=true.
Actually a second obscured event is triggered following by an
Unobscured, not an issue here.
In that mode the OS doesn't deactivate the app, allowing the app to run
under lock screen.
note: actually MonoGame doesn't allow run under lock screen
https://github.com/mono/MonoGame/pull/2084.

case #3
ApplicationIdleDetectionMode=Enabled|Disabled
App is suspended by a phone call.

Obscured event triggered. IsLocked=false.
We set IsActive=false on Obscure event.
When the action that obscured the app ends, an Unobscure event is
triggered and we set IsActive=true. No bug here.
That is similar to case #2. The OS doesn't deactivate the app.
Merge pull request #3020 from nkast/tncWP8FixObscureBug

Fix for obscure bug on WP8