apex manifest is always installed as apex_manifest.json

This change fixes a bug that when built with TARGET_FLATTEN_APEX=true,
apex manifests are installed without being renamed to
apex_manifest.json.

Test: TARGET_FLATTEN_APEX=true
Test: /system/apex/*/apex_mnifest.json exist
Test: device boots to the UI
Change-Id: Ib8baeb475babbf4faf6cc073e266bb3038ac1b2d