Fix(ads): Fix stopping ad manager after adblock.
Fix(ads): Fix stopping ad manager after adblock.

Previously, if the ad manager was shut down before the client side
ad manager finished setting up (most commonly due to adblock preventing
the ads metadata from being loaded), it would result in an error. The
client side ad manager assumed that the IMA ad manager would have been
loaded before unloading, and didn't check to see if the object was
null.
This prevents that error, by checking if the object is set before
stopping it.

Change-Id: Ic9e9b206bcd62641c0c2c6257b3eaacb6246e9df

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