Fix multithreaded backtraces for seccomp processes.

Add threads to the existing seccomp backtrace test to prevent
regressing this.

Bug: http://b/114139908
Bug: http://b/115349586
Test: debuggerd_test32
Test: debuggerd_test64
Change-Id: I07fbe1619b60f0008deb045a249f9045404478c2
4 files changed