APEX can be signed with different keys

A new module type 'apex_key' is defined to specify public and private
key pair for APEXs. An APEX can refer to the module via the property
'key'. When building the APEX, the private key from the key module is
used to sign it. In addition, the public key from the key module is
automatically installed to /system/etc/security/apex.

Bug: 115721587
Test: m apex.test; m
/apex/com.android.example.apex@1 exists

Change-Id: I82666db095bd7a09f6c1b9cbea2db57ebc076cbf