assmeble_vintf: IGNORE_TARGET_FCM_VERSION -> VINTF_*

All "private" environment variables to assemble_vintf
(inserted by the build rules, not build system variables)
should have this prefix (to distinguish against,
e.g. PRODUCT_ENFORCE_VINTF_MANIFEST)

Test: m framework_compatibility_matrix -j
Test: libvintf_test
Bug: 65028233
Change-Id: Iba44bd6d5d9a62b61666bcf5db672482d6f36488
1 file changed