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
Change-Id: I1da2f477f51b1d445bbee3b22a9c1fb268d25e80
6 files changed