bootanimation: Remove halfres zips before regenerating them

Change-Id: I8688700d85c3c29c650488cc469aed5ead5ec0e4
diff --git a/prebuilt/common/bootanimation/halfres/generate-half-res-anims.sh b/prebuilt/common/bootanimation/halfres/generate-half-res-anims.sh
index 2b6c9a9..a76ed5c 100755
--- a/prebuilt/common/bootanimation/halfres/generate-half-res-anims.sh
+++ b/prebuilt/common/bootanimation/halfres/generate-half-res-anims.sh
@@ -3,6 +3,7 @@
 HALF_RES_RESOLUTIONS="240 320 360 480 540 600 1440"
 
 for i in $HALF_RES_RESOLUTIONS; do
+	rm -f $i.zip
 	mkdir $i
 	cd $i
 	if [ -f ../../$(($i/2)).zip ]; then