Check "Secure start-up" before toggling emulation.

To fully emulate FBE we need to avoid the Cryptkeeper bounce, so
check to make sure user has disabled "Secure start-up" so vold can
unlock early during boot.

Bug: 27595501
Change-Id: Ic0d03f94982534381671e1039f14d1fd06bc0e4e

services/core/java/com/android/server/MountService.java

@@ -1946,6 +1946,10 @@ class MountService extends IMountService.Stub
                 throw new IllegalStateException(
                         "Emulation not available on device with native FBE");
             }
+            if (mLockPatternUtils.isCredentialRequiredToDecrypt(false)) {
+                throw new IllegalStateException(
+                        "Emulation requires disabling 'Secure start-up' in Settings > Security");
+            }
 
             final long token = Binder.clearCallingIdentity();
             try {