Fix warning about deprecated Gradle keepRules.file API (#7498)

* Fix warning about deprecated `keepRules.file` API

The proguard files have been moved to `app/proguard` in several directories for `common`, `foss`, and `enterprise` rules and it's those directories that are added through `keepRules.addStaticSourceDirectory(dir)`.

* Use `findPropery` instead of the deprecated `project.properties["key"]` API for `allWarningsAsErrors`

* Remove very legacy .gitignore rule and restore proguard `.keep` rules

app/build.gradle.kts

@@ -127,24 +127,11 @@ android {
             optimization {
                 enable = true
                 keepRules {
-                    files.add(File(projectDir, "common-proguard-rules.pro"))
-                    files.add(getDefaultProguardFile("proguard-android-optimize.txt"))
-
-                    // Depending on whether the app flavor is enterprise or not we want to use different proguard rules.
-                    val flavorProguardFile = if (isEnterpriseBuild) {
-                        // Custom rules for enterprise builds
-                        File(projectDir, "enterprise-proguard-rules.pro")
-                    } else {
-                        // These default rules prevent the OSS app from being obfuscated
-                        File(projectDir, "default-proguard-rules.pro")
-                    }
-
-                    if (flavorProguardFile.exists()) {
-                        files.add(flavorProguardFile)
-                    } else {
-                        logger.warn("Proguard file ${flavorProguardFile.absolutePath} does not exist")
-                    }
+                    // Equivalent of adding `getDefaultProguardFile("proguard-android-optimize.txt")` (this is the default value).
+                    includeDefault = true
                 }
+                // Our custom keep rules are registered as `keepRules` source folders in the `androidComponents` block below,
+                // as the former `keepRules.files` DSL is deprecated since AGP 9.
             }
         }
 
@@ -229,6 +216,30 @@ androidComponents {
     )
 
     onVariants { variant ->
+        // Register the R8 keep rules source folders for optimized build types (release, nightly).
+        // Replaces the deprecated `optimization.keepRules.files` DSL (AGP 9+).
+        if (variant.buildType != "debug") {
+            variant.sources.keepRules?.let { keepRules ->
+                // Common rules, always applied.
+                keepRules.addStaticSourceDirectory("proguard/common")
+
+                // Depending on whether the app flavor is enterprise or not we want to use different proguard rules.
+                val flavorProguardDir = if (isEnterpriseBuild) {
+                    // Custom rules for enterprise builds
+                    "../enterprise/proguard"
+                } else {
+                    // Custom fules for FOSS builds
+                    "proguard/foss"
+                }
+
+                if (File(projectDir, flavorProguardDir).exists()) {
+                    keepRules.addStaticSourceDirectory(flavorProguardDir)
+                } else {
+                    logger.warn("Proguard folder ${File(projectDir, flavorProguardDir).absolutePath} does not exist")
+                }
+            }
+        }
+
         // Assigns a different version code for each output APK
         // other than the universal APK.
         variant.outputs.forEach { output ->