Allow modules to be enabled/disabled depending on whether APEXes are flattened.

Allow the `enabled` Soong/Blueprint property (e.g. in `genrule` and
`cc_prebuilt_binary` module definitions) to have a different value
when APEX flattening is enabled.

Test: m checkbuild
Bug: 139277987
Change-Id: Ic586bdfac7690077445da12543154381d92cf98e
Merged-In: Ic586bdfac7690077445da12543154381d92cf98e