Enable prefab for fbjni integration (#1941)

Summary:
I'm in the progress of releasing a prefab-enabled FBJNI release. This dramatically reduces the hackery around integrating with separately released native libraries.

This will fail until the release is actually out.

Pull Request resolved: https://github.com/facebook/flipper/pull/1941

Test Plan:
`./gradlew :sample:installDebug` works as before.
Once the release is out, we need to check if this creates any problems with RN but because we advice to exclude the dependency, I expect it not to cause any trouble.

Reviewed By: mweststrate

Differential Revision: D26545104

Pulled By: passy

fbshipit-source-id: 1e21fa1816f28e2fcb52da68863fabc8bc625136

android/CMakeLists.txt

@@ -49,18 +49,12 @@ set(libflipper_build_DIR ${build_DIR}/libflipper/${ANDROID_ABI})
 set(libfolly_build_DIR ${build_DIR}/libfolly/${ANDROID_ABI})
 
 file(MAKE_DIRECTORY ${build_DIR})
-file(GLOB libfbjni_link_DIRS "${build_DIR}/fbjni*/jni/${ANDROID_ABI}")
-file(GLOB libfbjni_include_DIRS "${build_DIR}/fbjni-*-headers.jar/")
-
-# Without NO_CMAKE_FIND_ROOT_PATH, this will for some bizarre reason only look
-# in the NDK folder.
-find_library(FBJNI_LIBRARY fbjni PATHS ${libfbjni_link_DIRS} NO_CMAKE_FIND_ROOT_PATH)
+find_package(fbjni REQUIRED CONFIG)
 
 add_subdirectory(${libflipper_DIR} ${libflipper_build_DIR})
 
 target_include_directories(${PACKAGE_NAME} PRIVATE
     ${libjnihack_DIR}
-    ${libfbjni_include_DIRS}
     ${libflipper_DIR}
     ${libfolly_DIR}
     ${glog_DIR}
@@ -73,4 +67,4 @@ target_include_directories(${PACKAGE_NAME} PRIVATE
     ${LIBEVENT_DIR}/include/event2
     )
 
-target_link_libraries(${PACKAGE_NAME} ${FBJNI_LIBRARY} flippercpp)
+target_link_libraries(${PACKAGE_NAME} fbjni::fbjni flippercpp)