Fsync directories before delete key

The boot failure symptom is reproduced on Walleye devices. System boots
up after taking OTA and try to upgrade key, but keymaster returns "failed
to ugprade key". Device reboots to recovery mode because of the failure,
and finally trapped in bootloader screen. Possible scenario is:

(After taking OTA)
vold sends old key and op=UPGRADE to keymaster
keymaster creates and saves new key to RPMB, responses new key to vold
vold saves new key as temp key
vold renames temp key to main key -------------- (1) -- still in cache
vold sends old key and op=DELETE_KEY to keymaster
keymaster removes old key from RPMB ------------ (2) -- write directly to RPMB
==> SYSTEM INTERRUPTED BY CRASH OR SOMETHING; ALL CACHE LOST.
==> System boots up, key in RPMB is deleted but key in storage is old key.

Solution: A Fsync is required between (1) and (2) to cover this case.

Detail analysis: b/124279741#comment21

Bug: 112145641
Bug: 124279741
Test: Insert fault right after deleteKey in vold::begin (KeyStorage.cpp),
      original boot failure symptom is NOT reproducible.
Change-Id: Ib8c349d6d033f86b247f4b35b8354d97cf249d26