android build: Add reactNativeArchitectures Gradle property

This corresponds to two changes in the RN template app,
leading up to RN v0.68:

facebook/react-native@43c38cdc8 Allow specifying an architecture in RNTester and release builds
facebook/react-native@0f39a1076 Make the `reactNativeArchitectures` property more discoverable

Nothing will break without this, but it seems like potentially
a convenient feature sometimes -- speed up an iteration cycle
by limiting the build to just one architecture.  And including it
helps keep down the diff from the template app, slightly simplifying
future RN upgrades.

Author: gnprice

android/app/build.gradle

@@ -130,6 +130,14 @@ def jscFlavor = 'org.webkit:android-jsc-intl:+'
  */
 def enableHermes = project.ext.react.get("enableHermes", false);
 
+/**
+ * Architectures to build native code for.
+ */
+def reactNativeArchitectures() {
+    def value = project.getProperties().get("reactNativeArchitectures")
+    return value ? value.split(",") : ["armeabi-v7a", "x86", "x86_64", "arm64-v8a"]
+}
+
 android {
     defaultConfig {
         applicationId "com.zulipmobile"
@@ -164,7 +172,7 @@ android {
             reset()
             enable true // Build a separate APK for each ABI.
             universalApk false  // If true, also generate a universal APK
-            include "armeabi-v7a", "x86", "arm64-v8a", "x86_64"
+            include (*reactNativeArchitectures())
         }
     }
     buildTypes {

android/gradle.properties

@@ -29,3 +29,8 @@ android.enableJetifier=true
 
 # Version of flipper SDK to use with React Native
 FLIPPER_VERSION=0.99.0
+
+# Use this property to specify which architecture you want to build.
+# You can also override it from the CLI using
+# tools/gradle <task> -PreactNativeArchitectures=x86_64
+reactNativeArchitectures=armeabi-v7a,arm64-v8a,x86,x86_64