init: add MountHandler property handler

Set properties dev.mnt.blk.<mount_point>=<device_block_class> for mount
and umount operations by setting up an Epoll handler to catch
EPOLLERR or EPOLLPRI signals when /proc/mounts is changed.  Only
update properties associated with block devices.  For the mount
point of /, use the designation of /root instead.

Can use the properties in init rc expansion like:

on property dev.mnt.blk.root=*
    write /sys/block/${dev.mnt.blk.root}/queue/read_ahead_kb ${boot_read_ahead_kb:-2048}

on property dev.mnt.blk.data=*
    write /sys/block/${dev.mnt.blk.data}/queue/read_ahead_kb ${boot_read_ahead_kb:-2048}

on late-fs
    setprop boot_read_ahead_kb 128
    write /sys/block/${dev.mnt.blk.root}/queue/read_ahead_kb ${boot_read_ahead_kb}
    write /sys/block/${dev.mnt.blk.data}/queue/read_ahead_kb ${boot_read_ahead_kb}

Test: boot and inspect getprop results.
Bug: 124072565
Change-Id: I1b8aff44f922ba372cd926de2919c215c40ee874