bootanimation: Don't open non-existing bootanimation.zip

While booting from AOSP image, logcat always complains as following:
W/zipro   ( 1001): Unable to open zip '/data/local/bootanimation.zip':
No such file or directory
W/zipro   ( 1001): Unable to open zip '/system/media/bootanimation.zip':
No such file or directory

This patch avoids opening non-existing files.

Change-Id: I54cc03f125a5e16dbc930515bd2e43c623b63f8f