respect "apex_name:" for "apex" module

For now, apex_name is used only for flattened apex.
Even if apex_name is set, the activation point of unflattened is
determined by 'name' in apex_manifest.json.

This change make apex_name as priority.
If apex_name is set, then use this for apex name
- update name in apex_manifest.json to apex_name
- do not check if key filename matches (use apex_name as key name)

This can be useful if soong wants to rename apex module. Simply setting
apex_name has the same effect of renaming "activation point" of apex.

But: 139774701
Test: m (soong test)
Change-Id: I8ea3645e4aa8f317997bc1443ec308ed0595b1c2