Build VNDK with VNDK prebuilts without BOARD_VNDK_VERSION

Current VNDK prebuilts are disabled if BOARD_VNDK_VERSION is not set,
but VNDK prebuilts should be enabled even BOARD_VNDK_VERSION is not set
because VNDK APEXes should be available from VNDK deprecation. This
change removes some restrictions on VNDK prebuilts to enable those from
VNDK deprecation.

Bug: 316829758
Test: AOSP Cuttlefish build succeeded without BOARD_VNDK_VERSION
Change-Id: Id780811dab26f2125097c3efc5b2b4a59416b826