IpSecManager and IpSecAlgorithm API Tweaks

-Add a reserveSecurityParamterIndex() function that allows the
 system to select an SPI.
-Disallow INVALID_SECURITY_PARAMETER_INDEX from being passed as
 an explicit SPI request.
-Remove the ALGO_ prefix from constants in IpSecAlgorithm

Bug: 36073210
Test: Updated CTS tests still pass on bullhead
Change-Id: Ic94809996076b0718f153f550b82192fe7048a2e