Removed unsafe use of hidl_vec<>.setToExternal

hidl_vec objects that do not own their associated buffer are highly
unsafe in multithreaded environments where move semantic is used to
transfer ownership between threads. With keystore transitioning to a
multi threaded execution model we can no longer use this optimization
safely.

Bug: 111443219
Test: Ran full keystore cts test suite.
Change-Id: I9a366fc7df5dfee508dc092855545963ef6d9665