ReactAndroid: JS errors during bundle load were reported as UnknownCppException (#24648)

Summary:
This fixes a regression on Android introduced by f3e5cce where JS errors thrown during bundle load were lost (shown only as UnknownCppException). It is especially tough to debug (custom) bundling errors without seeing the javascript error.

Root cause hypothesis: since switching to clang, `JSError`s thrown in ReactCommon's `JSCRuntime::checkException` were never matched as `std::exception` in ReactAndroid's `convertCppExceptionToJavaException` due to these missing flags.

I'm a bit shy on low-level details concerning how C++ rtti works exactly around catching exceptions thrown in other libraries. All I can say that with this change, a `bundle.android.js` that only contains `throw new Error("wtf");` now nicely outputs a message and stack trace in the logcat. Before (and since DanielZlotin's switch to clang) it just outputted:

```
2019-04-29 12:17:59.365 1162-1306/com.rntest E/unknown:ReactNative: Exception in native call
    com.facebook.jni.UnknownCppException: Unknown
        at com.facebook.react.bridge.queue.NativeRunnable.run(Native Method)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at com.facebook.react.bridge.queue.MessageQueueThreadHandler.dispatchMessage(MessageQueueThreadHandler.java:29)
        at android.os.Looper.loop(Looper.java:214)
        at com.facebook.react.bridge.queue.MessageQueueThreadImpl$4.run(MessageQueueThreadImpl.java:232)
        at java.lang.Thread.run(Thread.java:764)
```

[Android] [Fixed] - JS errors during bundle load were reported as UnknownCppException.
Pull Request resolved: #24648

Differential Revision: D15123525

Pulled By: mdvacca

fbshipit-source-id: 74b5ce9ebae38d172446b6e31739d795c601947b

Author: EmielM

ReactAndroid/src/main/jni/Application.mk

@@ -27,7 +27,7 @@ NDK_MODULE_PATH := $(APP_MK_DIR)$(HOST_DIRSEP)$(THIRD_PARTY_NDK_DIR)$(HOST_DIRSE
 APP_STL := c++_shared
 
 # Make sure every shared lib includes a .note.gnu.build-id header
-APP_CFLAGS := -Wall -Werror
+APP_CFLAGS := -Wall -Werror -fexceptions -frtti
 APP_CPPFLAGS := -std=c++1y
 APP_LDFLAGS := -Wl,--build-id