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
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 2e9062e..e04db5d 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -804,6 +804,13 @@
     sp<Fence> mPreviousPresentFence = Fence::NO_FENCE;
     bool mHadClientComposition = false;
 
+    enum class BootStage {
+        BOOTLOADER,
+        BOOTANIMATION,
+        FINISHED,
+    };
+    BootStage mBootStage;
+
     struct HotplugEvent {
         hwc2_display_t hwcDisplayId;
         HWC2::Connection connection = HWC2::Connection::Invalid;
@@ -824,7 +831,6 @@
     nsecs_t mLastSwapBufferTime;
     volatile nsecs_t mDebugInTransaction;
     nsecs_t mLastTransactionTime;
-    bool mBootFinished;
     bool mForceFullDamage;
     bool mPropagateBackpressure = true;
     std::unique_ptr<SurfaceInterceptor> mInterceptor =