Make `BaselineProfileRule` experimental.

Fixes: b/203692160
Test: N/A
Relnote: Adds a new experimental `BaselineProfileRule` which is capable of generating baseline profiles for app's critical user journey.
Change-Id: Ibbefa8ef27e60b6634f9c4a604db91ae715c69c5
diff --git a/benchmark/benchmark-macro-junit4/build.gradle b/benchmark/benchmark-macro-junit4/build.gradle
index 6ca1488..77d6df4 100644
--- a/benchmark/benchmark-macro-junit4/build.gradle
+++ b/benchmark/benchmark-macro-junit4/build.gradle
@@ -16,6 +16,7 @@
 
 import androidx.build.LibraryGroups
 import androidx.build.Publish
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
     id("AndroidXPlugin")
@@ -61,3 +62,10 @@
     inceptionYear = "2020"
     description = "Android Benchmark - Macrobenchmark JUnit4"
 }
+
+// Allow usage of Kotlin's @OptIn.
+tasks.withType(KotlinCompile).configureEach {
+    kotlinOptions {
+        freeCompilerArgs += [ '-Xopt-in=kotlin.RequiresOptIn' ]
+    }
+}