Fix MediaHTTPConnection.disconnect() blocking for a long time.

MediaHTTPConnection.seek() was creating new connections in a while
loop without checking whether another thread was busy concurrently
disconnecting.

When a new connection was created between the time the disconnect()ing
thread had disconnected the old one and acquired the synchronized
block, the new connection wouldn't be disconnected and therefore
seek() would not encounter an IOException; therefore, seek() would
not return quickly, leaving the disconnecting thread waiting to
acquire the synchronized block for a long time.

This CL fixes this by making seek() throw IOException quickly if
it discovers that another thread is trying to disconnect. This
is checked shortly before and after the new connection is created,
to avoid a race based on the order between the new connection
being created and the disconnecting thread reading the old
connection value. Note that this still doesn't stop a new
connection being created shortly after the previous one was torn
down - it only stops the disconnecting thread waiting for a long
time to acquire the synchronized lock.

Fixes: 131894499
Test: The following command hang 3/3 times before this CL, but
      succeeded 3/3 times afterwards:
      atest android.media.cts.NativeDecoderTest#testAMediaDataSourceClose

Change-Id: I3862a4367d0e46c64c0cbf7bcaa369aca5692871