Create rule to generate the exportable api files when checkapi is disabled

WITHOUT_CHECK_API environment variable enables the checkapi to be
removed from the critical path. The variable is rarely used within AOSP,
but the downstream vendors may depend on it, in order to improve the
build performance given that the api signature file through metalava is
a resource costly task.

Currently, the exportable api files / removed api files are not
generated when checkapi is disabled, but a module may depend on the
exportable api files when a checkapi is disabled. In order to prevent
the missing build rules error in this case, generate the rule to copy
the checked in api file / removed api file to the exportable api file /
removed api file to prevent build errors.

This change also fixes the error message when the OutputFiles(string)
api file is null, to correctly inform the user the error.

Test: m BUILD_FROM_SOURCE_STUB=true WITHOUT_CHECK_API=true && inspect ninja path and verify that "non-updatable-exportable-current.txt" depends on the generated exportable api file.
Bug: 329374072
Change-Id: I24f88d450fb46b6ea9d5920d83617d8228edd34b