BLASTSyncEngine: Avoid overlapping syncs.

WindowState is missing a condition in prepareForSync that
WindowContainer already has about not joining a new sync when
an old one is in progress. We fix this by extracting the code
in-to a shared method.

Bug: 156758771
Test: WindowOrganizerTests
Change-Id: Ia7e9ba6b1263ccce27e7245b332669763104ae94