Do not compare KEEP_VNDK with false

Overall android build system assumes empty value as false, so comparing
KEEP_VNDK value with false can cause unexpected problem in the future.
It should be changed to compare with true in opposite way.

Bug: 299197187
Test: aosp_cf build succeeded with KEEP_VNDK=false
Change-Id: I5db685c2735309a14ea6bdb8b8928b2aa987929f