Fixes a potential error in setDynamicShortcut

In the current implementation, ret.complete will still be called after
ret.completeExceptionally is called. Although the second call doesn't do
anything right now since the implementation of AndroidFuture ignores
consecutive attempts at calling complete on an already completed future,
it is better to avoid these kind of behaviors in the first place.

Bug: 186237458
Test: manual
Change-Id: I65e44c28ded947d499ba74aafd9a2686f02a9f18