Fix inconsistencies in the apex names used for the apex variations.

Add an apex_name property to prebuilt APEX modules to allow specifying
the "runtime" name of the APEX, i.e. the one it gets mounted as in /apex/,
which is also the one used for the apex variations.

Introduce a callback to retrieve that name consistently for all APEX
modules (apex, override_apex, prebuilt_apex, and apex_set), and update
some apex mutator code paths to use it.

For APEX source modules (apex and override_apex), it's necessary to add
a new field in apexBundle, since the name property gets overridden for
the override variant that override_apex creates.

Test: m nothing
Bug: 191269918
Change-Id: If8612639bffdf91cbcab3387b0603bf5dffef1f5