Fix vr flinger post thread to resume correctly

After being paused, the vr flinger post thread remained paused until it
was enabled by requesting display ownership, but the request for display
ownership was made from the vr flinger post thread, causing the post
thread to get stuck in the paused state forever. I moved the display
ownership request to the vr flinger dispatcher thread, so the post
thread can be resumed.

Bug: 62666511

Test: Put a device through a sleep/wake cycle and confirmed the vr
flinger post thread resumes as expected.

Change-Id: Ic57adb7376c8f2b5760bfaa4a5a30b58894a7ab7