SF: Initialize all displays on boot/restart

Generalize SF::initializeDisplays (called on boot and restart) to:
    - Apply the transaction that clears DisplayState to all displays.
    - Power on all displays.

The first change removes a special case for the primary display, setting
the stage for multi-display boot animation. Each display is assigned its
own LayerStack, and set up with a projection to its active resolution.

The second change fixes a bug where DisplayCapability::BRIGHTNESS was
not detected for secondary displays present during boot. SF queries
capabilities when a display is first powered on, but DM asks SF about
brightness when the display is hotplugged, regardless of power mode.
The general fix (covering external displays) is for DM to defer its
query, but this stopgap covers internal displays.

Revert I3a2eae4efc4a5c6113700a9ca9e9b261e364a878, which let the initial
power mode be std::nullopt. This effectively forced DM's first request
to setPowerMode(<rear display>, OFF), which would otherwise be ignored
because OFF had been the default power mode on DisplayDevice creation.
However, that special case confusingly took the same branch as the OFF
to ON transition, and is no longer needed now that all displays are ON
(from SF's perspective, not just HWC's) until the boot animation ends.

Fixes: 267633741
Fixes: 150889228
Bug: 269510347
Test: Boot unfolded and folded.
Test: Induce system_server crash.
Test: InitializeDisplaysTest.initializesDisplays
Change-Id: I5277a629f39b3b285452aa84d49ff84e3dc957ca