Eliminate power manager latency for boot completed.

The power manager disables the power button until boot completed
occurs.  If there are many pending broadcasts in the queue, it
may be possible for BOOT_COMPLETED to be delayed for several
seconds after boot.

To avoid the delay, introduced a new boot phase which is
sent to system services immediately when boot completed happens.

Bug: 13398280
Change-Id: I1833d2ffb20305009dd76363b43e534034f1d0a2