rust: Add rust_ffi_static vendor ramdisk Support

Similar to our vendor support, this adds support for linking rust static
libraries to vendor ramdisk cc modules.

A bug fix is also included where a restriction against setting rust_ffi
vendor-specific was not being enforced.

Bug: 179397942
Test: Example modules link, Soong tests pass.
Change-Id: I737cdf0c2f49ab349bcea2a0429e6298ebc1313e

rust/testing.go

@@ -101,6 +101,7 @@ func GatherRequiredDepsForTest() string {
 			no_stdlibs: true,
 			host_supported: true,
 			vendor_available: true,
+			vendor_ramdisk_available: true,
                         native_coverage: false,
 			sysroot: true,
 			apex_available: ["//apex_available:platform", "//apex_available:anyapex"],
@@ -113,6 +114,7 @@ func GatherRequiredDepsForTest() string {
 			no_stdlibs: true,
 			host_supported: true,
 			vendor_available: true,
+			vendor_ramdisk_available: true,
                         native_coverage: false,
 			sysroot: true,
 			apex_available: ["//apex_available:platform", "//apex_available:anyapex"],