Support AllowMissingDependencies in prebuilt_apex modules

Arch-specfic prebuilt_apex modules may be missing prebuilts for new
architectures.  Mark the module as missing dependencies and allow
build analysis to continue when AllowMissingDependencies is set.

Bug: 250918230
Test: lunch aosp_riscv64-userdebug && m ALLOW_MISSING_DEPENDENCIES=true nothing
Change-Id: I79f0c20a0c9443fe443f9ed9bc846aa649f6b6a6