Revert "ANDROID: fixup! 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 f049c419.

Bug: 63142211
Change-Id: I83c42123921e8dd0395420125e69246c4d85cda6
Signed-off-by: Greg Hackmann <ghackmann@google.com>