Return unique_ptr from readFromParcel

When KeyCharacterMap is read from parcel, there's no need to force the
caller to store the object in a shared pointer. We can return a
unique_ptr first, and let the caller decide on how exactly that lifetime
should be managed.

Bug: 274058082
Test: presubmit
Change-Id: I8c5ec1e32a9304f6ad186bc0279f4c7bcbab77d8