Don't prompt at boot if we already did that when decrypting

vold will store password securely until KeyGuard requests it
and hands it on to KeyStore.

Needs matching vold changes from
 https://googleplex-android-review.git.corp.google.com/#/c/432050/

Bug: 12990752
Change-Id: I930ed8180cf0b8feb1e58db043d5fb6dff1bab20