build apexkeys.txt

apexkeys.txt is a text file having APEX-to-keys mappings. The file is
included in the target-files package where it is used when re-sign the
APEXes with release keys.

Each line of the file consists of 5 fields:
1) name: file name of the APEX
2) public_key: the public key for the apex_payload.img of the APEX
3) private_key: the private key used to sign the apex_payload.img
4) container_certificate: the certificate used to sign the APEX zip
container
5) container_private_key: the private key used to sign the APEX zip
container

Bug: 124406181
Test: m out/soong/apexkeys.txt and inspect the content
Test: TARGET_BUILD_APPS=com.android.tzdata m dist and make sure
out/dist/apexkeys.txt exists
Change-Id: I1daa13ec50956323b97e15e8df7f1fbe5ea21d63