Look for bootanimation in apex-module

Look for bootanimation.zip in /apex/com.android.bootanimation/etc/.
This is needed to support download and install of customized bootanimations.

Bug: 116821733
Test:
1. Get a custom bootanimation.zip
2. Build a com.android.bootanimation.apex that contains the custom
   bootanimation.
3. Build a product partition containing the public avb-key in
   /product/etc/security/apex/ or a system partition containing the
   public avb-key in /system/product/etc/security/apex/
4. Flash the partition that was built.
5. $ adb install com.android.bootanimation.apex
6. $ adb reboot
7. Verify that the custom bootanimation is used during boot.

Change-Id: I0b63e8dd2ba536a5077e34f63670025f6e9d791a