Android template: Allow overriding default "index.js" entry file (#26769)

Summary:
- Using "System.getenv" allows to specify any entry file using environment variables and without modifying gradle file. Example:

    export ENTRY_FILE="another_entry_file.js"
    ./gradlew assembleDebug

- This functionality is also more align with iOS implementation that uses 'if [[ "$ENTRY_FILE" ]]; then'. See #23667 for more details.

- Possibility to define entry file on CI without modifying sources (Example: project like [pixels-catcher](https://www.npmjs.com/package/pixels-catcher) requires different entry file)

## Changelog:

[Android] [Added]  - Custom entry file on android using `ENTRY_FILE` environment variable
Pull Request resolved: #26769

Test Plan:
- Create a project from template

- Define `ENTRY_FILE` environment variable

```
export ENTRY_FILE="anotherIndexFile.js"
```

- Build android

```
./gradlew assembleDebug
```

Expected result: App contains bundle file that starts from `anotherIndexFile.js` file.

Differential Revision: D17903165

Pulled By: cpojer

fbshipit-source-id: 6b7cdf229918d101c170aa5fbdca6f3ef293d41c

Author: Maksym Rusynyk

react.gradle

@@ -7,10 +7,22 @@ import org.apache.tools.ant.taskdefs.condition.Os
 
 def config = project.hasProperty("react") ? project.react : [];
 
+def detectEntryFile(config) {
+    if (System.getenv('ENTRY_FILE')) {
+        return System.getenv('ENTRY_FILE')
+    } else if (config.entryFile) {
+        return config.entryFile
+    } else if ((new File("${projectDir}/../../index.android.js")).exists()) {
+        return "index.android.js"
+    }
+
+    return "index.js";
+}
+
 def cliPath = config.cliPath ?: "node_modules/react-native/cli.js"
 def composeSourceMapsPath = config.composeSourceMapsPath ?: "node_modules/react-native/scripts/compose-source-maps.js"
 def bundleAssetName = config.bundleAssetName ?: "index.android.bundle"
-def entryFile = config.entryFile ?: "index.android.js"
+def entryFile = detectEntryFile(config)
 def bundleCommand = config.bundleCommand ?: "bundle"
 def reactRoot = file(config.root ?: "../../")
 def inputExcludes = config.inputExcludes ?: ["android/**", "ios/**"]
@@ -149,7 +161,7 @@ afterEvaluate {
                             hermesFlags = config.hermesFlagsDebug
                             if (hermesFlags == null) hermesFlags = []
                         }
-                        
+
                         if (Os.isFamily(Os.FAMILY_WINDOWS)) {
                             commandLine("cmd", "/c", getHermesCommand(), "-emit-binary", "-out", hbcTempFile, jsBundleFile, *hermesFlags)
                         } else {

template/android/app/build.gradle

@@ -15,7 +15,9 @@ import com.android.build.OutputFile
  *   // the name of the generated asset file containing your JS bundle
  *   bundleAssetName: "index.android.bundle",
  *
- *   // the entry file for bundle generation
+ *   // the entry file for bundle generation. If none specified and
+ *   // "index.android.js" exists, it will be used. Otherwise "index.js" is
+ *   // default. Can be overridden with ENTRY_FILE environment variable.
  *   entryFile: "index.android.js",
  *
  *   // https://facebook.github.io/react-native/docs/performance#enable-the-ram-format
@@ -76,7 +78,6 @@ import com.android.build.OutputFile
  */
 
 project.ext.react = [
-    entryFile: "index.js",
     enableHermes: false,  // clean and rebuild if changing
 ]