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

android/config.go

@@ -516,6 +516,10 @@ func (c *deviceConfig) BtConfigIncludeDir() string {
 	return String(c.config.ProductVariables.BtConfigIncludeDir)
 }
 
+func (c *deviceConfig) DeviceKernelHeaderDirs() []string {
+	return c.config.ProductVariables.DeviceKernelHeaders
+}
+
 func (c *deviceConfig) NativeCoverageEnabled() bool {
 	return Bool(c.config.ProductVariables.NativeCoverage)
 }