trusty/storage: Add property indicating when fs is ready

Adds a system vendor property (ro.vendor.trusty.storage.fs_ready) to
indicate when backing storage on the Android filesystem (e.g. /data) is
ready for use. Before this property is set, the Trusty storage proxy may
restart causing connections in Trusty to the storage service to be
disconnected. All Trusty operations that may require storage and can
wait until the device filesystems are ready should wait on this
property.

Bug: 258018785
Test: manual
Change-Id: I9b1408b72df34a0d0cbcc1b99e9617f15bc47558