Use consistent naming for allocating SPI.

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