Add manifest kernel FCM version

Add target-level attribute to manifest.kernel. A device manifest
can specify this attribute to specify the kernel requirements which
FCM version it conforms to.

This CL adds parsing code for the target-level attribute.

Test: libvintf_test
Bug: 139309488

Change-Id: Ibd29e0778432d046e06345d8c6e22656773819c2
diff --git a/KernelInfo.cpp b/KernelInfo.cpp
index 4f3215e..ec8abac 100644
--- a/KernelInfo.cpp
+++ b/KernelInfo.cpp
@@ -28,6 +28,10 @@
     return mConfigs;
 }
 
+Level KernelInfo::level() const {
+    return mLevel;
+}
+
 bool KernelInfo::matchKernelConfigs(const std::vector<KernelConfig>& matrixConfigs,
                                     std::string* error) const {
     for (const KernelConfig& matrixConfig : matrixConfigs) {