Use consistent naming for allocating SPI.

Throughout the IPsec code (API, system server, netd) we use "reserve"
SPI and "allocate" SPI interchangeably. This renames to use "allocate"
everywhere for self-consistency and consistency with the kernel
(ALLOCSPI).

In javadoc, I am leaving the word "reserve" in several places because it
is still an accurate description of how the method behaves.

Bug: 69128142
Test: TreeHugger should be enough
Change-Id: I8ea603b4612303b0393beef04032671fa53d2106