SingleWriterBpfMap - introduce getSingleton with caching am: 52d6c5587f

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3134198

Change-Id: I53854b7b5779871d05b6754633ff36a5cb8837e4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>