Merge "Mark stable and document android.system.OsConstants" am: 63c49f31e4 am: 7f11b5a277 am: cbd7b7e51a

Original change: https://android-review.googlesource.com/c/platform/libcore/+/1652282

Change-Id: Ib383dbbfdea2c92e25cb93fcedc79bd63dd146bf