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/proguard/common/common-proguard-rules.keep

@@ -0,0 +1,79 @@
+# Add project specific ProGuard rules here.
+# You can control the set of applied configuration files using the
+# proguardFiles setting in build.gradle.kts.
+#
+# For more details, see
+#   http://developer.android.com/guide/developing/tools/proguard.html
+
+# Increase optimizations passes to 3
+-optimizationpasses 3
+
+# JNA
+-dontwarn java.awt.*
+-keep class com.sun.jna.** { *; }
+-keep class * implements com.sun.jna.** { *; }
+
+# TagSoup, coming from the RTE library
+-keep class org.ccil.cowan.tagsoup.** { *; }
+
+# kotlinx.serialization
+
+# Kotlin serialization looks up the generated serializer classes through a function on companion
+# objects. The companions are looked up reflectively so we need to explicitly keep these functions.
+-keepclasseswithmembers class **.*$Companion {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+# If a companion has the serializer function, keep the companion field on the original type so that
+# the reflective lookup succeeds.
+-if class **.*$Companion {
+  kotlinx.serialization.KSerializer serializer(...);
+}
+-keepclassmembers class <1>.<2> {
+  <1>.<2>$Companion Companion;
+}
+
+# OkHttp platform used only on JVM and when Conscrypt and other security providers are available.
+# Taken from https://raw.githubusercontent.com/square/okhttp/master/okhttp/src/jvmMain/resources/META-INF/proguard/okhttp3.pro
+-dontwarn okhttp3.internal.platform.**
+-dontwarn org.conscrypt.**
+-dontwarn org.bouncycastle.**
+-dontwarn org.openjsse.**
+
+# Needed for Posthog
+-keepclassmembers class android.view.JavaViewSpy {
+    static int windowAttachCount(android.view.View);
+}
+
+# Keep LogSessionId class and related classes (https://github.com/androidx/media/issues/2535)
+-keep class android.media.metrics.LogSessionId { *; }
+-keep class android.media.metrics.** { *; }
+
+# Keep Media3 classes that use reflection (https://github.com/androidx/media/issues/2535)
+-keep class androidx.media3.** { *; }
+-dontwarn android.media.metrics.**
+
+# New rules after AGP 8.13.1 upgrade
+-dontwarn androidx.window.extensions.WindowExtensions
+-dontwarn androidx.window.extensions.WindowExtensionsProvider
+-dontwarn androidx.window.extensions.area.ExtensionWindowAreaPresentation
+-dontwarn androidx.window.extensions.layout.DisplayFeature
+-dontwarn androidx.window.extensions.layout.FoldingFeature
+-dontwarn androidx.window.extensions.layout.WindowLayoutComponent
+-dontwarn androidx.window.extensions.layout.WindowLayoutInfo
+-dontwarn androidx.window.sidecar.SidecarDeviceState
+-dontwarn androidx.window.sidecar.SidecarDisplayFeature
+-dontwarn androidx.window.sidecar.SidecarInterface$SidecarCallback
+-dontwarn androidx.window.sidecar.SidecarInterface
+-dontwarn androidx.window.sidecar.SidecarProvider
+-dontwarn androidx.window.sidecar.SidecarWindowLayoutInfo
+
+# Also needed after AGP 8.13.1 upgrade, it seems like proguard is now more aggressive on removing unused code
+-keep,allowoptimization,allowshrinking class org.matrix.rustcomponents.sdk.** { *;}
+-keep,allowoptimization,allowshrinking class uniffi.** { *;}
+-keep,allowoptimization,allowshrinking class io.element.android.x.di.** { *; }
+
+# Keep Metro classes
+-keep,allowoptimization,allowshrinking class dev.zacsweers.metro.** { *; }
+
+# Rustls Platform Verifier
+-keep, includedescriptorclasses class org.rustls.platformverifier.** { *; }