Correctly install APK-in-APEX

APK in a flattened APEX is installed as 'ETC' class module, instead of
the 'APP' class. This is to prevent Make from doing app-specific
amendments (e.g. such as adding module name after my_module_path)
to the paths and filenames which are all correctly set in the Soong
side.

Test: add `installable:true` to development/samples/Snake/Android.mk
and add 'Snake' to the `apps` property of an APEX. The build is
successful.
Test:  build the APEX on a device with TARGET_FLATTEN_APEX=true
The APK is at system/apex/<apexname>/app/Snake/Snake.apk

Bug: 142537672

Change-Id: I314bf1ab7abd4d4a4e9fa210442c004f54d8ccca