Don't reboot into recovery if block map file is missing.

We added a third parameter to RecoverySystem.installPackage() to let the
caller to indicate the package has been processed (uncrypt'd). We need
to ensure the caller's claim is true by checking the existence of the
block map. Otherwise the device will fail for sure when booting into
recovery.

Bug: 27620932
Change-Id: I6325455253480055f14eb0cf020689ac37328602