Make bootanim multi-display aware

Also, some refactoring of existing code.

Flag: com.android.graphics.bootanimation.flags.multidisplay
Bug: 269510347
Test: Manual (build, flash, reboot in folded and unfolded states)
Change-Id: Ieb360f47cc38cab90b1eca471b0e773e303f88e2
diff --git a/cmds/bootanimation/bootanimation_flags.aconfig b/cmds/bootanimation/bootanimation_flags.aconfig
new file mode 100644
index 0000000..04837b9
--- /dev/null
+++ b/cmds/bootanimation/bootanimation_flags.aconfig
@@ -0,0 +1,12 @@
+package: "com.android.graphics.bootanimation.flags"
+container: "system"
+
+flag {
+  name: "multidisplay"
+  namespace: "bootanimation"
+  description: "Enable boot animation on multiple displays (e.g. foldables)"
+  bug: "335406617"
+  is_fixed_read_only: true
+}
+
+