Expose folly flags and make folly_runtime available to be consumed via prefab

Summary:
This removes the old way of consuming `libfolly_runtime.so` from
`Android-prebuilt.cmake` to using Prefab which is natively supported
by the Android pipeline.

Moreover, I've exposed a `folly-flags.cmake` file which can be used by libraries
to know which lists of flags they need to use for Folly.

Changelog:
[Internal] [Changed] - Move `folly_runtime` to be consumed via prefab

Reviewed By: cipolleschi

Differential Revision: D39928571

fbshipit-source-id: 4864c37813ec681a6a464c97de6dfc078f5553f1

ReactAndroid/cmake-utils/Android-prebuilt.cmake

@@ -41,26 +41,6 @@ target_include_directories(fb
         INTERFACE
         ${FIRST_PARTY_NDK_DIR}/fb/include)
 
-## folly_runtime
-add_library(folly_runtime SHARED IMPORTED GLOBAL)
-set_target_properties(folly_runtime
-        PROPERTIES
-        IMPORTED_LOCATION
-        ${REACT_NDK_EXPORT_DIR}/${ANDROID_ABI}/libfolly_runtime.so)
-target_include_directories(folly_runtime
-        INTERFACE
-        ${THIRD_PARTY_NDK_DIR}/boost/boost_1_76_0
-        ${THIRD_PARTY_NDK_DIR}/double-conversion
-        ${THIRD_PARTY_NDK_DIR}/folly)
-target_compile_options(folly_runtime
-        INTERFACE
-        -DFOLLY_NO_CONFIG=1
-        -DFOLLY_HAVE_CLOCK_GETTIME=1
-        -DFOLLY_HAVE_MEMRCHR=1
-        -DFOLLY_USE_LIBCPP=1
-        -DFOLLY_MOBILE=1
-        -DFOLLY_HAVE_XSI_STRERROR_R=1)
-
 ## react_nativemodule_core
 add_library(react_nativemodule_core SHARED IMPORTED GLOBAL)
 set_target_properties(react_nativemodule_core