Improve stats_event memory usage

We now truncate the buffer to the appropriate length when clients call
stats_event_build().

Benchmarking tests indicate that truncating the buffer to the
appropriate length increases the cost clients pay to write to the socket
by 2%. This is negligible enough that I decided to truncate the buffer
for both pushed and pulled atoms in order to simplify the API.

Test: m libstatssocket
Test: bit libstatssocket_benchmark:*
Bug: 144126231
Change-Id: I35dec748ff87c0821d0d06779a406997e6e64966
Merged-In: Ife976bb383ecff8de5064730692a95e2a3a82c9d