Revert "Revert "[LLDB][GUI] Expand selected thread tree item by default""

This reverts commit fd18f0e8.

I reverted this change to see its effect on failing GUI tests on LLDB
Arm/AArch64 Linux buildbots. I could not find any evidence against this
particular change so reverting it back.

Differential Revision: https://reviews.llvm.org/D100243