Partial revert of "Remove ConnectivityServiceTest signature perms use"

This reverts the Android.bp changes in commit
bba8ec0d43ff4c9219f259bdfe1cc4f05faa609b.

Reason for revert: This broke ConnectivityServiceTest (https://android-build.googleplex.com/builds/tests/view?invocationId=I90700009364410436&testResultId=TR01525128446370769)

A full revert has a merge conflict in the other file, and is likely
not necessary to fix the failure.

Change-Id: Icc0c2500590a6b936016574be4f234b9f748cb80
diff --git a/tests/unit/Android.bp b/tests/unit/Android.bp
index 468fa0b..ad59c94 100644
--- a/tests/unit/Android.bp
+++ b/tests/unit/Android.bp
@@ -54,6 +54,7 @@
     ],
     platform_apis: true,
     test_suites: ["device-tests"],
+    certificate: "platform",
     jarjar_rules: "jarjar-rules.txt",
     static_libs: [
         "androidx.test.rules",