Refactor product inheritance for shiba products

Add an additional layer of inheritance for `aosp_shiba`,
`aosp_shiba_fullmte`, and `aosp_shiba_pgagnostic`.

This change adds a common layer of inheritance (shiba_generic.mk)
for non-factory products. This makes it easier to configure factory
and non-factory products differently.

Non-factory `shiba` products now inherits from
`device/google/shusky/shiba_generic.mk` which inherit from
`device/google/shusky/device-shiba.mk`.

aconfig flags: Flag: EXEMPT mechanical refactoring
build flags: Flag: EXEMPT mechanical refactoring

Bug: 332219400
Bug: 335567293
Test: lunch aosp_shiba-trunk_staging-userdebug && m
      lunch aosp_shiba_fullmte-trunk_staging-userdebug && m
      lunch aosp_shiba_pgagnostic-trunk_staging-userdebug && m

Change-Id: Ib825430eaf56c1395c0d955a4bbc46e5512f14b8
2 files changed