odrefresh: move file-system utilities into odr_fs_utils.{h,cc}

Preparation for metrics support which will use the same functionality.

Add tests for odrefresh file-system utilities too.

Bug: 160683548
Test: atest art_odrefresh_tests

(cherry picked from commit dda82d2ed4d073866f3c3da3d88ad178261c62f4)
Merged-In: I1da2f477f51b1d445bbee3b22a9c1fb268d25e80
Change-Id: If4b8cfdb9c68167aae07268b29140e1162af2293
6 files changed