clarify setProtectedVm req message

I didn't notice protectedVm vs mProtectedVm vs mProtectedVmSet
immediately. Due to my failure to notice this difference, I
spent 10 extra minutes.

Bug: 254912288
Test: N/A
Change-Id: I2b5eac85e27344b39dbff753ff1fbb0b7cca85a4
diff --git a/javalib/src/android/system/virtualmachine/VirtualMachineConfig.java b/javalib/src/android/system/virtualmachine/VirtualMachineConfig.java
index 3061f65..b016669 100644
--- a/javalib/src/android/system/virtualmachine/VirtualMachineConfig.java
+++ b/javalib/src/android/system/virtualmachine/VirtualMachineConfig.java
@@ -353,7 +353,7 @@
             }
 
             if (!mProtectedVmSet) {
-                throw new IllegalStateException("protectedVm must be set explicitly");
+                throw new IllegalStateException("setProtectedVm(t/f) must be called explicitly");
             }
 
             if (mProtectedVm