Move pdx_tool into system_ext

it is not a part of the system according to
generic_system.mk.

Bug: 173538619
Bug: 167689596
Test: presubmit
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: I06d22d1086db330a4e56ae2f0939c4ef04683e82