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 1ba6b2f4.

Bug: 63142211
Change-Id: I4a1472c3efeaf74d11c678a0b92e1687c5f43702
Signed-off-by: Greg Hackmann <ghackmann@google.com>