add log mechanism for Android port

[jeffzhou2000]

No description provided.

[jeffzhou2000]

I rollback the local master branch, then updated latest source code from upstream's master branch, then updated the remote master branch of my forked repo; then created a new work branch "log_to_android_1 " for this PR.

so everything in my working repo(including this working branch "log_to_android_1") seems clean now.

[jeffzhou2000]

1. To enable log output in Android system, add rs2::log_to_console(...) in JNI_OnLoad or other appropriate place;

jint JNI_OnLoad(JavaVM *vm, void *reserved) {
rs2::log_to_console(RS2_LOG_SEVERITY_DEBUG); //the param RS2_LOG_SEVERITY_DEBUG will ouput detailed log in librealsense internal, pls modify it accordingly
...
...
...
return JNI_VERSION_1_4;
}

2. To disable log output thoroughly in Android system, don't call rs::log_to_console(...);

[dorodnic]

The PR will be auto-completed once dev is merged with master after release