Change reserveSecurityParameterIndex() to take a remoteAddress

To make the SPI reservation more semantically consistent with the
transform creation API, and to ensure that we always create SPI
reservations relative to a well-known remote, we should take the
SPI request relative to a remote (rather than to a destination).
This necessitates that we now consider direction separately, which
is used for keying the SA-Id.

Bug: 36073210
Test: compilation
Change-Id: I81e955c20128c1f8e04fd68eb26669561f827a78