Apply snapshot variant to vndk package module name

VNDK snapshot phony package, vndk_v$VER, is redefined to
separate phony packages per VNDK snapshot build variant,
vndk_v$VER_$VARIANT, where $VARIANT is defined as the primary
$TARGET_ARCH of the build target.

Accordingly, top level Android.bp file under prebuilts/vndk/v$VER
is redefined as separate Android.bp files for each snapshot
variant at prebuilts/vndk/v$VER/$VARIANT/Android.bp.

Test: m -j PRODUCT_EXTRA_VNDK_VERSIONS=27 (after installing
snapshot to prebuilts/vndk/v27)
Bug: 71370248

Change-Id: Ida449978090fe8a4f16376c991d34fa0945be596