ANDROID: Revert "GKI: build in a kernel command line of "rw""

This reverts commit cc47b201.

Bug: 173425293

Reason for revert: Android platform has adapted devices with and
without a dedicated recovery partition, so the ramdisk no longer
needs to be writable.

Change-Id: Ib743e94197eadccbe877811e3a78aaa969960ea8
Signed-off-by: Yifan Hong <elsk@google.com>