Add matrix.kernel.level.

Indicates at which level is the kernel requirement from.

Also fix tests now that matrix.kernel.level is serialized.

Bug: 139309488
Test: vintf_object_test
Test: libvintf_test

Change-Id: Ic65b0a2a95a2311b480753a139f3029e87848786
diff --git a/MatrixKernel.cpp b/MatrixKernel.cpp
index 0caaec5..30953d6 100644
--- a/MatrixKernel.cpp
+++ b/MatrixKernel.cpp
@@ -27,10 +27,8 @@
     return true;
 }
 
-bool MatrixKernel::setSourceMatrixLevel(Level level) {
-    if (mSourceMatrixLevel != Level::UNSPECIFIED) return false;
+void MatrixKernel::setSourceMatrixLevel(Level level) {
     mSourceMatrixLevel = level;
-    return true;
 }
 
 Level MatrixKernel::getSourceMatrixLevel() const {