Only debuggable packages can be downgraded.

This reflects the change in PackageManager service:
frameworks/base commit 921dd754ab49df0cd580ff96503f7616c4c85f4a.

Bug: 27327503
Change-Id: Ic13788a3850c4d1a3a566b208853de9a6d2c5237
diff --git a/cmds/pm/src/com/android/commands/pm/Pm.java b/cmds/pm/src/com/android/commands/pm/Pm.java
index 24449d4..4025553 100644
--- a/cmds/pm/src/com/android/commands/pm/Pm.java
+++ b/cmds/pm/src/com/android/commands/pm/Pm.java
@@ -1490,7 +1490,7 @@
         System.err.println("    -i: specify the installer package name");
         System.err.println("    -s: install application on sdcard");
         System.err.println("    -f: install application on internal flash");
-        System.err.println("    -d: allow version code downgrade");
+        System.err.println("    -d: allow version code downgrade (debuggable packages only)");
         System.err.println("    -p: partial application install");
         System.err.println("    -g: grant all runtime permissions");
         System.err.println("    -S: size in bytes of entire session");