Use pipes instead of tmp files.

Dumpstate hal does not have selinux permissions to files under
/data/local/tmp. So we pass fds to pipes to the hal for testing, which
works out because of this selinux rule:

allow domain su:fifo_file { write getattr };

Bug: 67648445
Test: VtsHalDumpstateV1_0Target
Change-Id: I804ef03250f18d3aa34393297e8ac9725e437dbb