Use default (testkey) cert for SimAppDialog.

SimAppDialog does not require any special permissions; it is launched
by the platform (controlled by the
config_carrierAppInstallDialogComponent resource) and only needs to
launch the user's app store to install a carrier app.

Fixes: 162396906
Test: TreeHugger; will flash/verify functionality on TH build
Change-Id: I1ed1202b4f080f690b816b99219de45164aab8fe

packages/SimAppDialog/Android.bp

@@ -4,7 +4,6 @@ android_app {
     srcs: ["src/**/*.java"],
 
     platform_apis: true,
-    certificate: "platform",
 
     static_libs: [
         "androidx.legacy_legacy-support-v4",