hwui : fix memory leak due to duplicate in shadow cache

New ShadowTask with the ShadowDescription key that already
exists in the shadow LruCache will leak as it is not being
added.

Fix adds check for the existing key that is common in the hwui
code but missing for the TessellationCache::precacheShadow
function.

Change-Id: I37fd5ec82f8b8da5d1ec0f2ab9fd04c5f8534367