Added ResumeManager so something can be drawn whilst textures are reloaded

**How it works**
The textures are now reloaded on a different thread so execution can continue. The AndroidGameWindow._isResuming is set so the optional IResumeManager can be drawn. A default ResumeManager implementation has been provided so users can easily set it up with a custom texture rotating in the middle of the screen.

**Motivation**
Reloading of textures can take a LONG time. Whilst this occurs the user is presented with a black screen and the music playing. This leads to confusion about whether the game may have crashed. Also, when adding full screen ads to a game, these generally cause the GL context to be lost so when the user dismisses the ad, they are hit with this lengthy texture reload each time.