Make apex_set support ":<apex>" syntax to access the apex file

The apex and prebuilt_apex module types make the output APEX available
using ":<apex>" syntax, e.g. for use as a data file in a test. This
change adds compatible behavior to apex_set to ensure consistent
behavior when building against prebuilts.

Bug: 259958590
Test: m MODULE_BUILD_FROM_SOURCE=false nothing
Change-Id: Ic6f105976cfdd6af4920cc1e11ab0b4ff72e6a6f