Revert "ANDROID: 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 2956c9be.

Bug: 63142211
Change-Id: I9ff2728531eb2b09b5ddb19da51eacf6e4316dd6
Signed-off-by: Greg Hackmann <ghackmann@google.com>