libcore: update OWNERS

Update team members.

Mark some approvers as last resort (see go/android-codereviews).

Comment out preferred default reviewer as syntax not supported by
checkowners.py nor respected by gerrit.

Bug: N/A
Test: build/make/tools/checkowners.py -c libcore/OWNERS
Change-Id: I9459701283acbf1274ae304faf512717f41d911b
1 file changed