Make package installer tests post submit

As they are UI-tests and hence to flaky for pre-submit.

Fixes: 131353722
Test: n/a
Change-Id: I7bea06bbe16717406ccd0676fb83396066959a92
diff --git a/packages/PackageInstaller/TEST_MAPPING b/packages/PackageInstaller/TEST_MAPPING
index 42aa47c..5d7b9bb 100644
--- a/packages/PackageInstaller/TEST_MAPPING
+++ b/packages/PackageInstaller/TEST_MAPPING
@@ -1,5 +1,5 @@
 {
-  "presubmit": [
+  "postsubmit": [
     {
       "name": "CtsPackageInstallTestCases",
       "options": [