app: a bench build type for P0's benchmark gate

Own application id (.bench suffix) and label ("AI Sessions bench" via a
build-type resValue over the new @string/app_name), release
optimisations, signed with the same key build-apk.sh already generates,
FIXTURE_MODE=true wired through BuildConfig. Its asset source set points
straight at app/bench-fixture/assets rather than a copy under androidApp,
so there is one file to keep in sync with the generator, not two.

build-apk.sh bench builds it; the CA-pinning step is untouched and still
requires a real ca.pem to exist, even though this build never connects --
simplest to let it pin whatever is there rather than special-casing it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

app/androidApp/src/main/AndroidManifest.xml

@@ -25,7 +25,7 @@
          the fix is a judgement about how this app should look. Drop this
          suppression when a real icon lands. -->
     <application
-        android:label="AI Sessions"
+        android:label="@string/app_name"
         android:allowBackup="true"
         android:theme="@android:style/Theme.Material.Light.NoActionBar"
         tools:ignore="MissingApplicationIcon">

app/androidApp/src/main/res/values/strings.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <!-- Overridden by the `bench` build type's resValue (build.gradle.kts) to "AI Sessions bench",
+         so the two are never mistaken for each other in the launcher or in Settings. -->
+    <string name="app_name">AI Sessions</string>
+</resources>