Path to external storage directory is hardcoded

[TarCV]

fork/fork-runner/src/main/java/com/shazam/fork/system/io/RemoteFileManager.java

Line 29 in 4a05c2a

private static final String FORK_DIRECTORY = "/sdcard/fork";

According to Android linter the path to external storage mount directory should not be hardcoded.
For example IDevice#getMountPoint(MNT_EXTERNAL_STORAGE) can be used instead.
I have actually encountered devices where adb shell echo $EXTERNAL_STORAGE returns a path different from /sdcard.

I think it's possible that this issue causes videos not being recorded like in #138

[AndreiZaitcev]

This should be resolved by ee92adf