Get RNTester to compile and run

Summary:
This diff includes a few changes:
1. Move the headers inside `jsiexecutor` into `jsiexecutor/jsireact`. As far as I'm aware, the Android ndk build system isn't flexible enough to support header namespaces, so we can't just expose the headers inside the `jsiexecutor` directory under the `jsireact` namespace. Therefore, I moved the headers to `jsiexecutor/jsireact`, and added `jsiexecutor` to the header search path.  This was the easiest way to simulate `jsireact` namespace.
2. Setup the Android.mk files to get RNTester compiling and running.
3. Introduce a `jscexecutor` module to make `JSCExecutor.java` execute without throwing.

**Note:** Moving the header files inside `jsiexecutor` probably breaks the iOS builds and internal builds. I'll fix those in subsequent diffs on this stack.

Reviewed By: shergin

Differential Revision: D9995429

fbshipit-source-id: 418a4ee91f585842c5e317af2f300227a51e9ba8