Fix data race in RecentsLoaderTests

Fix data race in RecentsLoaderTests#testContentsUpdate_observable() that would occur when accessing `private boolean mContentChanged` variable from the Main (a.k.a. UI) thread and the thread the test runs on.
Instead of relying on a class member mContentChanged use the existing thread-safe CountDownLatch object.

Bug: 288342044
Test: atest --rerun-until-failure 10000 \
      DocumentsUIUnitTests:com.android.documentsui.RecentsLoaderTests
Change-Id: I336ecddf4c894d1ba383a4246e7eb4a58ac200e2
Merged-In: I336ecddf4c894d1ba383a4246e7eb4a58ac200e2
(cherry picked from commit f243c48d)