VNDK is fully deprecated from Soong

VNDK is fully deprecated, so KeepVndk variable will have fixed value.
This change removes KeepVndk config value, and updates any logic using
the value as VNDK is always deprecated.

Bug: 330100430
Test: Soong tests passed
Ignore-AOSP-First: Resolve merge conflict
Change-Id: I98b7590c059883e06bf3fb236d88966de64991d7