stats: Add a vendor atom reporting interface

This interface will allow a client to upload vendor atoms through the
IStats HAL. The allowed data types will be limited to int32_t, int64_t,
float, and string. The atom ID must be in the reserved non-AOSP OEM
range specified by statsd.

Test: su 0 ./stats_client -v
Bug: 122541417
Change-Id: I768a32e77b59ef414ae6a9525533bda2ad3b5fc2
Signed-off-by: Maggie White <maggiewhite@google.com>