Fix fallback paths for dumping threads.

In the fallback path, if the non-main thread is the target
to be dumped, then no other threads are dumped when creating
a tombstone. Fix this and add unit tests to verify that
this all threads, including the main thread are dumped.

Bug: 234058038

Test: All unit tests pass.
Test: debuggerd -b media.swcodec process
Test: debuggerd media.swcodec process
Change-Id: Ibb75264f7b3847acdbab939a66902d986c0d0e5c