HLS: Queue discontinuity only for the first block

If a segment jump happens in HLS, the discontinuity gets
queued for every block read. This leads to continuous decoder
flushes and is observed as a long video freeze.

Changes done to queue the discontinuity only for the first block
after segment jump.

CRs-Fixed: 696452

Change-Id: I79409fe552611fcf3cb6ea9cbc76c8d4f9f5541f
(cherry picked from commit 965459f9)