Fixed blinking header on a quick swipe

[TigerHix]

On a quick swipe between the tabs, due to a weird behavior of Android - onPageSelected will sometimes be triggered before onPageScrolled - the expected behavior:

...
Android calls: onPageSelected(0)
Android calls: onPageSelected(0)
Android calls: onPageSelected(1)
Android calls: onPageSelected(1)
...

would become:

...
Android calls: onPageSelected(0)
Android calls: onPageSelected(0)
Android calls: onPageSelected(1)
Android calls: onPageSelected(0) // ???
Android calls: onPageSelected(1)
Android calls: onPageSelected(1)
Android calls: onPageSelected(1)
...

Hence the header image "blinks". Thanks to: https://stackoverflow.com/questions/13819865/how-to-know-viewpager-has-scrolled-half-the-screen-in-android/30662470#30662470, this problem is now resolved.