Fix system server leaked fd dump

'lsof' was execed to dump the open file descriptors, but it does
not have the same permissions as system_server, presumably due to
SELinux restrcitions.

Now we get a list of open file descriptors from /proc/<pid>/fd and
resolve them via readlink, all inside system_server.

Test: Manually tested on a device and compared with the output of adb
lsof.

Change-Id: I4db8369a3a2bb0009c776277a8a6f92f72857ef8