Switch LocalSocket to android::base::{Send,Receive}FileDescriptorVector.

The previous implementation allocated an array of size
CMSG_SPACE(count) to store CMSG_LEN(count * sizeof(int)) elements, which
leads to bad things happening for values of count greater than 1 on
32-bit, and 2 on 64-bit.

Test: atest android.net.LocalSocketTest
Test: atest android.net.cts.LocalSocketTest
Change-Id: I0a9502c3358d8fa92d2d20e344c6270d6baedc07