Build the payload image only during apk-in-apex signing

When doing apk-in-apex signing, the package name is not reserved
during the apex repacking. As a result, the name accidentally
reverts to 'com.android.wifi' from 'com.google.android.wifi'.

This cl changes the behavior to call 'apexer' by passing the
'--payload_only' argument. So we don't build the apex file from
scratch and the old AndroidManifest.xml will be reused.

Test: 152084536
Bug: unit tests pass
Change-Id: I8332b2ee84832fb196f2e1c4309abac5ab92e153