RNTester Android: always compile Fabric files

Summary:
This commit makes both `:ReactAndroid` and `:rn-tester:android:app` always compile in Fabric codegen outputs. However, one may still enable/disable Fabric at runtime by setting `USE_FABRIC` env var (set to 1 or 0, default is 0).

Note that we can't register custom components specific to the app, yet, so only the components in react-native github repo is covered by this commit.

RNTester doesn't enable Fabric by default yet due to known UI bugs that haven't been addressed yet.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D25674311

fbshipit-source-id: 8db660c959319250ebc683c84076677cf6489e94

ReactAndroid/build.gradle

@@ -33,11 +33,6 @@ def thirdPartyNdkDir = new File("$buildDir/third-party-ndk")
 //   - glog-0.3.5
 def dependenciesPath = System.getenv("REACT_NATIVE_DEPENDENCIES")
 
-// The 'USE_FABRIC' environment variable will build Fabric C++ code into the bundle
-// USE_FABRIC=0 will build RN excluding fabric
-// USE_FABRIC=1 will build RN including fabric
-def enableFabric = (System.getenv('USE_FABRIC') ?: '0').toBoolean()
-
 // The Boost library is a very large download (>100MB).
 // If Boost is already present on your system, define the REACT_NATIVE_BOOST_PATH env variable
 // and the build will use that.
@@ -318,7 +313,6 @@ def buildReactNdkLib = tasks.register("buildReactNdkLib", Exec) {
             "REACT_COMMON_DIR=$projectDir/../ReactCommon",
             "REACT_GENERATED_SRC_DIR=$buildDir/generated/source",
             "REACT_SRC_DIR=$projectDir/src/main/java/com/facebook/react",
-            "BUILD_FABRIC=$enableFabric",
             "-C", file("src/main/jni/react/jni").absolutePath,
             "--jobs", project.findProperty("jobs") ?: Runtime.runtime.availableProcessors()
     )

ReactAndroid/src/main/jni/react/jni/Android.mk

@@ -134,10 +134,7 @@ include $(REACT_SRC_DIR)/reactperflogger/jni/Android.mk
 # TODO (T48588859): Restructure this target to align with dir structure: "react/nativemodule/..."
 # Note: Update this only when ready to minimize breaking changes.
 include $(REACT_SRC_DIR)/turbomodule/core/jni/Android.mk
-
-ifeq ($(BUILD_FABRIC),true)
-  include $(REACT_SRC_DIR)/fabric/jni/Android.mk
-endif
+include $(REACT_SRC_DIR)/fabric/jni/Android.mk
 
 # TODO(ramanpreet):
 #   Why doesn't this import-module call generate a jscexecutor.so file?