Harcode partition info of apk-in-apex for apkcerts.txt

`apex` creates an androidmk entry for each apk-in-apex, but does not
add its partition info to `LOCAL_*_SPECIFIC` of these autogenerated
androidmk modules. Therefore partition of apk-in-apex in make is always
"system".

This CL hardcodes the partition of these modules to "system" when
building with Soong as well.

Bug: 399788149
Test: Verified that these diff disappears
< name="CompOSPayloadApp.apk" certificate="build/make/target/product/security/testkey.x509.pem" private_key="build/make/target/product/security/testkey.pk8" partition="system"

Change-Id: I8a0d32e69fae8a30e7612a0e017d218b6db5f083
---
> name="CompOSPayloadApp.apk" certificate="build/make/target/product/security/testkey.x509.pem" private_key="build/make/target/product/security/testkey.pk8" partition="system_ext"
Test: The files are now bit-identical