Add enable_desktop_indicator_in_separate_thread_bugfix flag.

Adding bugfix flag per discussion at https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/base/+/30771569/comment/adfcedbc_2ae776ba/

Bug: 366413536
Test: m
Flag: EXEMPT, adding flag
Change-Id: Iaffcc6b9d176844727f540cb76d1e7c374774fc8