Update UT cases for CarrierAppUtils

The CL includes the following changes:
- Introduce aconfig flag to protect the fix
- Add a new test case to cover the case when carrier app has
  been uninstalled before, it should not be uninstalled again.
- Add more space lines for each test case to improve readability
  (with given/when/then style).
- Update comment of test cases to make them more comprehensive

Bug: 18205685
Test: atest CarrierAppUtilsTest
Change-Id: I30c5f65c8dac55d6a44862bc4a4ad5f256e17056
diff --git a/flags/misc.aconfig b/flags/misc.aconfig
index c9816b9..51f10cd 100644
--- a/flags/misc.aconfig
+++ b/flags/misc.aconfig
@@ -93,3 +93,13 @@
         purpose: PURPOSE_BUGFIX
     }
 }
+
+flag {
+    name: "hide_preinstalled_carrier_app_at_most_once"
+    namespace: "telephony"
+    description: "Fix bug when preloaded carrier app is uninstalled and lose provisioning data"
+    bug:"158028151"
+    metadata {
+        purpose: PURPOSE_BUGFIX
+    }
+}