Fixes full paths on Android (cleaner)

If one adds a search resolutions order directory without a trailing slash, CCFileUtils::getFullPathForDirectoryAndFilename will fail on Android. For example it will return:

assets/sdBg1.png

Instead of the expected:

assets/sd/Bg1.png

This small commit fixes the function to add a slash if necessary, fixing the issue on Android.

Compared to the last pull request of this name, formatting matches the style of code around it and comments have been added.
Merge pull request #3447 from NatWeiss/patch-5

Fixes full paths on Android (cleaner)