Clear closed captions map when text engine is destroyed
Clear closed captions map when text engine is destroyed

Change-Id: I610334e6ad187a09e8831cde285d9a0e38876aad

shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player