Clear Selected Row's Background When User Navigates away, Fixes #18

- Added `resetBackground()` method in `BrowseFragment` to reset the background image to the default image, `amphitheater.png`
- Overwrote `onResume` method in `BrowseFragment` and called `resetBackground()` in it so when user navigates away from the `DetailsActivity` the background is reset to the default one.
- Defined an interface in `BrowseActivty`  and implemented it in `BrowseFragements`
- In `BrowseActivity` overwrote `onBackPressed()` to call the interface's method so the background is reset to the default when the user presses the back menu.

To Test.
Select an item in the row and see the background change, navigate away from it by pressing the back menu and see the default background show
nit: space after `if`