The transport system API needs to manage binder identity

...around writing settings.  It does its own proper permission check;
it just needs to make sure not to accidentally crash the caller in
strange and wondrous ways because of failing to clear binder identity
before writing the result to secure settings.

Bug 16542048

Change-Id: I88d1f2dbeebd24eed5d86989f0ca0d834878b054