Add RBE environment variables config example and documentation.

Test: N/A
Change-Id: Ic80f3f192e8e3449f02d8c99058c37e97251a4d2

docs/rbe.json

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+{
+    "env": {
+        "USE_RBE": "1",
+
+        "RBE_R8_EXEC_STRATEGY": "remote_local_fallback",
+        "RBE_CXX_EXEC_STRATEGY": "remote_local_fallback",
+        "RBE_D8_EXEC_STRATEGY": "remote_local_fallback",
+        "RBE_JAVAC_EXEC_STRATEGY": "remote_local_fallback",
+        "RBE_JAVAC": "1",
+        "RBE_R8": "1",
+        "RBE_D8": "1",
+
+        "RBE_instance": "[replace with your RBE instance]",
+        "RBE_service": "[replace with your RBE service endpoint]",
+
+        "RBE_DIR": "prebuilts/remoteexecution-client/live",
+
+        "RBE_use_application_default_credentials": "true",
+
+        "RBE_log_dir": "/tmp",
+        "RBE_output_dir": "/tmp",
+        "RBE_proxy_log_dir": "/tmp"
+    }
+}

docs/rbe.md

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+# Build Android Platform on Remote Build Execution
+
+Soong is integrated with Google's Remote Build Execution(RBE) service, which
+implements the
+[Remote Executaion API](https://github.com/bazelbuild/remote-apis).
+
+With RBE enabled, it can speed up the Android Platform builds by distributing
+build actions through a worker pool sharing a central cache of build results.
+
+## Configuration
+
+To enable RBE, you need to set several environment variables before triggering
+the build. You can set them through a
+[environment variables config file](https://android.googlesource.com/platform/build/soong/+/master/README.md#environment-variables-config-file).
+As an example, [build/soong/docs/rbe.json](rbe.json) is a config that enables
+RBE in the build. Once the config file is created, you need to let Soong load
+the config file by specifying `ANDROID_BUILD_ENVIRONMENT_CONFIG_DIR` environment
+variable and `ANDROID_BUILD_ENVIRONMENT_CONFIG` environment variable. The
+following command starts Soong with [build/soong/docs/rbe.json](rbe.json)
+loaded:
+
+```shell
+ANDROID_BUILD_ENVIRONMENT_CONFIG=rbe \
+ANDROID_BUILD_ENVIRONMENT_CONFIG_DIR=build/soong/doc \
+  build/soong/soong_ui.bash
+```
+
+### Configuration Explanation
+
+Below a brief explanation of each field in
+[build/soong/docs/rbe.json](rbe.json):
+
+##### USE\_RBE:
+If set to 1, enable RBE for the build.
+
+##### RBE\_CXX\_EXEC\_STRATEGY / RBE\_JAVAC\_EXEC\_STRATEGY / RBE\_R8\_EXEC\_STRATEGY / RBE\_D8\_EXEC\_STRATEGY:
+
+Sets strategies for C++/javac/r8/d8 action types. Available options are
+(**Note**: all options will update the remote cache if the right permissions to
+update cache are given to the user.):
+
+*   **local**: Only execute locally.
+*   **remote**: Only execute remotely.
+*   **remote_local_fallback**: Try executing remotely and fall back to local
+    execution if failed.
+*   **racing**: Race remote execution and local execution and use the earlier
+    result.
+
+##### RBE\_JAVAC / RBE\_R8 / RBE\_D8
+
+If set to 1, enable javac/r8/d8 support. C++ compilation is enabled by default.
+
+##### RBE\_service / RBE\_instance
+
+The remote execution service endpoint and instance ID to target when calling
+remote execution via gRPC to execute actions.
+
+##### RBE\_DIR
+
+Where to find remote client binaries (rewrapper, reproxy)
+
+##### RBE\_use\_application\_default\_credentials
+
+reclient uses
+[application default credentials](https://cloud.google.com/sdk/gcloud/reference/auth/application-default/login)
+for autentication, as generated by `gcloud auth application-default login`
+
+##### RBE\_log\_dir/RBE\_proxy\_log\_dir/RBE\_output\_dir
+
+Logs generated by rewrapper and reproxy will go here.