Merge bluetooth apex default in it's only usage

This is to avoid confusion on witch value is apply, the one from default
or the one from the apex or the one from the vendor override if any.

Rules are:
* module `apex` and module `apex_default` are merged (like in this CL)
* module `override_apex` keep all existing values as long as the key is
  not repeated inside the `override_apex`. If it is, discard all change
  in the base apex (or its default)

Bug: 230359868
Test: adb install <apex>; and check the files on phone
Tag: #refactor
Ignore-AOSP-First: No apex on aosp
Change-Id: Ie9adfc04dbe0920db8ce8d6aa756ca91a3101865