ReactAndroid: add -fexceptions -frtti to jni CFLAGS

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, JSErrors thrown in
ReactCommon's JSCRuntime::checkException were never matched as
std::exception in Android's convertCppExceptionToJavaException due
to these missing flags.

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