Revert "proc: make oom adjustment files user read-only"

CTS no longer expects oom_{adj,score_adj} to be read-only.  See
https://android-review.googlesource.com/530687/

 for additional context.

This reverts commit 55541f1a.

Bug: 63142211
Change-Id: I8011b4389b17d9577a6aff08943d0021e990171b
Signed-off-by: Greg Hackmann <ghackmann@google.com>