Add a method to get the pointer to the ring buffer am: d66eb85122

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/libfmq/+/22433300

Change-Id: I7b017ea2c828b23cb274275ff594c266ab995f0c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>