add device_kernel_headers module for vendor-specific kernel headers

device_kernel_headers is a built-in heder-only lib that provides device-specific
kernel headers. The header path is configured via a new product variable
SystemIncludeDirs, which is currently the mirror of
TARGET_PROJECT_SYSTEM_INCLUDES in the make world.

Note: generic kernel headers (bionic/libc/kernel) have been added to the
include path by default. "device_kernel_headers" module is for device-specific
kernel headers such as /device/*/*/kernel-headers.

Note 2: this is opt-in for Android.bp modules (i.e. header_libs :
["device_kernel_headers"] required.) while it is always provided to
Android.mk modules.

Bug: 62939405
Test: choosecombo to aosp_sailfish (or any other Pixel/Nexus targets)
BOARD_VNDK_VERSION=current m -j gralloc.msm8996 (or any other vendor
libs using vendor-specific kernel headers)

Change-Id: I81c60abc13942c89fff723d1544b27a81b300db0
diff --git a/Android.bp b/Android.bp
index 1eac394..496dcd2 100644
--- a/Android.bp
+++ b/Android.bp
@@ -141,6 +141,8 @@
         "cc/ndk_sysroot.go",
 
         "cc/llndk_library.go",
+
+        "cc/kernel_headers.go",
     ],
     testSrcs: [
         "cc/cc_test.go",
@@ -308,3 +310,8 @@
         },
     },
 }
+
+kernel_headers {
+    name: "device_kernel_headers",
+    vendor: true,
+}