Fix race condition in MediaHTTPConnection

getSize() and getMIMEType() rely on seekTo() having completed.
If seekTo() was called on a different thread, for example because
NuCachedSource2 was doing reads on a background thread, getSize()
could return -1 if it was called after connection was established,
but before the response had been parsed.

Bug: 110230427
Test: manual
Change-Id: I469b47e025f1c7a7ef3123d3347668f0541e6f2d