Add StrongPointer::release()

Useful when dealing with raw pointers in unavoidable places. Avoids
an awkward (and "slow") dance of mySp->incStrong(0); mySp->get(); ~mySp;

Test: make && atest --host libutils_binder_test
Change-Id: Ib8d46150592725cc256779bccfed19a16dce78b2