Add option to redirect native stdout and stderr to Logcat

[mhsmith]

Chaquopy already redirects the Python stdout and stderr to the Android logcat. However, the native stdout and stderr are occasionally useful for debugging the Python startup process, or seeing error messages produced by non-Python libraries.

In order to capture any output from the Python startup process, the redirection would have to be done (or at least controlled) by AndroidPlatform. We shouldn't enable this by default, because it might conflict with other native code that's doing the same thing.

The native streams can be redirected using this technique, and the commented-out code in chaquopy_java.pyx for redirection to a file should be removed.