add ndk atrace sample

[jayshah123]

Native tracing sample is non-trivial as mentioned here: https://developer.android.com/topic/performance/tracing/custom-events#native-code

and would be a useful kickstart sample

[ggfan]

acknowledged: probably having a codelab to include the whole process of enabling tracing and viewing the trace. There has been updates about tracing and profiling in Studio IDE, need to do some digging; will put this on the schedule to follow up, so many thing happening, may not get to this right away.