Restrict Emergency Assist API to system image only.

Bug: 23591361
Change-Id: Idecae74384568e9cb45e605b6eeadad9dea367ab
(cherry picked from commit 66229fd15cf42c0f09169122cc8a245f64569965)
diff --git a/src/com/android/phone/EmergencyActionGroup.java b/src/com/android/phone/EmergencyActionGroup.java
index 2d70d01..039dd82 100644
--- a/src/com/android/phone/EmergencyActionGroup.java
+++ b/src/com/android/phone/EmergencyActionGroup.java
@@ -165,11 +165,9 @@
                 } catch (PackageManager.NameNotFoundException e) {
                     continue;
                 }
-                // Get earliest installed app, but prioritize system apps.
-                if (bestMatch == null
-                        || !isSystemApp(bestMatch) && isSystemApp(packageInfo)
-                        || isSystemApp(bestMatch) == isSystemApp(packageInfo)
-                                && bestMatch.firstInstallTime > packageInfo.firstInstallTime) {
+                // Get earliest installed system app.
+                if (isSystemApp(packageInfo) && (bestMatch == null ||
+                        bestMatch.firstInstallTime > packageInfo.firstInstallTime)) {
                     bestMatch = packageInfo;
                 }
             }