trace-ipc: Don't hold procLock while calling out.

Dumping IPC transactions can be a long running operation; instead of
holding the procLock for all of it, make a copy of the list of
processes, and iterate over those instead.

Bug: 306025653
Flag: EXEMPT bugfix
Test: adb shell am trace-ipc start / stop
Change-Id: I4986b70e020528ef12afe7a6f94835604025c1c1