trusty/storage: Add property indicating when fs is ready am: 4762c22aba am: 9294177936

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/20600926

Change-Id: Ib4bdc67666c15d6838793c213fefaa5dd4f32069
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>