Add an additional layer of inheritance for `aosp_comet`.
This change adds a common layer of inheritance for non-factory
products (comet_generic.mk). This makes it easier to configure
factory and non-factory products differently.
Non-factory `comet` products now inherits from
`device/google/comet/comet_generic.mk` which inherit from
`device/google/comet/device-comet.mk`.
aconfig flags: Flag: EXEMPT mechanical refactoring
build flags: Flag: EXEMPT mechanical refactoring
Bug: 336452008
Bug: 336707612
Test: lunch aosp_comet-trunk_staging-userdebug && m
Change-Id: I22306fbe0eadc217d61a19ebd163265f9d0f12a0
