Create InflateMessageDocumentHolder on UI thread during test

Previously this was attempted to be done by decorating the test with
@UiThreadTest, but that doesn't seem to work because the test runner
isn't the AndroidJUnit4 runner. Instead call out to runOnMainSync do do
so.

Bug: 423126739
Test: atest 'DocumentsUIGoogleTests:com.android.documentsui.dirlist.InflateMessageDocumentHolderTest#testClickingButtonShouldShowProgressBar'
Flag: TEST_ONLY
Change-Id: I2cb9b26001ace74bfd0a98dae8feba632c3454f0