adb: switch apacket payload to a type that doesn't initialize its contents.

Switch from using std::string as the type we use to hold our payload in
apacket to a custom reimplementation that doesn't zero initialize. This
improves bulk transfer throughput in the adb_benchmark microbenchmark
on walleye by ~20%.

Test: adb shell taskset f0 /data/benchmarktest64/adb_benchmark/adb_benchmark
Change-Id: Ibad797701eb1460c9321b0400c5b167b89b2b4d0