Do not broadcast VOLUME_STATE_CHANGED before boot has completed.

This is for avoiding unnecessary launch of processes which receive
VOLUME_STATE_CHANGED (e.g. com.android.externalstorage) duting boot.
This change improves boot time by ~200 ms.

BUG: 22163689
Change-Id: I76d39695552d49b6188bad8fdae81c5744013396