surfaceflinger: signalRefresh after boot animation starts

Assume BootStage::BOOTLOADER initially.  Switch to
BootStage::BOOTANIMATION on first buffer latch and switch to
BootStage::FINISHED after bootFinished is called.

Do not invoke signalRefresh when in BootStage::BOOTLOADER.  We do
not want to replace bootloader splash by a blank screen.  This saves
HWC from workarounds that may or may not work reliably.

Bug: 79434305
Bug: 110772452
Test: reboot and observe
Change-Id: I9e892e629303177431acd2cfe23f0f984ca6866e