Remove duplicate directory variables with the same value

Bugreport files and screenshot files are saved in the same directory.
Use common variable for consistency and to avoid duplicacy.

Move bugreport file and default screenshot file creation to the
constructor as for each bugreportInfo object a bugreportFile must exist.

Bug:123617758
Test: Takes interactive/full bugreports as expected
Change-Id: I338e852c543e63b403536cd852518b6601ea32f0