[NAN] Remove builder API using String to translate to byte[]

Per API review: String accessor was not needed (duplicate, semantics
not accurate, trivial use-case).

(cherry-pick of commit c76ccf3e)

Bug: 31470256
Test: unit tests & integration (sl4a) testing.
Change-Id: Id307702627afd8ee579b540d4c2f65359bf9a925