sepolicy: mount firmware partitions with firmware_file context

* we don't need to give ueventd access across the entire vfat context

Change-Id: Idf1e24b7b90fd439d1d3ee2cd839c43078f026c7
a5: update hardware keys overlay

Add volume rocker to device keys.

Change-Id: Ifc12387a18de9f476d3ea5cab3ea7812bfab77f4