Don't use prebuilt_apex when TARGET_FLATTEN_APEX is set to true

When the device does not support non-flattened APEX (i.e.
TARGET_FLATTEN_APEX is set to true), then ignore the prebuilt_apex
modules even when `prefer: true`.

Bug: 136251130
Bug: 136662772
Test: build marlin, and check if /system/apex has flattened APEXes
Change-Id: I9f3dfefc3bd357d6750fbb1e418937d095720b04