Connected displays wait for boot complete

Sending display "connected" for external display
too early during boot may result into
ExternalDisplayStatsService won't be able to
register listeners due to other services
for example ActivityManager may not be available
so early in the boot. Also, there is a problem
with disabling display early in the boot which
results into display not possible to re-enable
upon user click on Mirror dialog.

This CL postpones the disabling the connected
display together with initializing ExternalDisplay
StatsService until boot completes.

Bug: 328463830
Bug: 318595765
Test: restart device while external display is connected
Test: atest ExternalDisplayPolicyTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:66b273e12d79b59478d17205982b428aecc29edd)
Merged-In: I472a82cfe8db01b644b14c05bab90f1de91818e8
Change-Id: I472a82cfe8db01b644b14c05bab90f1de91818e8