Device-specific kernel headers are accessbile again

Device-specific kernel headers (TARGET_PROJECT_SYSTEM_INCLUDES) have
been added to the global include paths. However, since global include
paths are no longer provided when a lib is built for vendors, the
device-specific kernel headers becomes inaccessible. To solve this
problem, a pseudo header library 'device_kernel_headers' is defined
and it is added to all Android.mk defined lib/bins.

Also, in order to give this info to the soong world,
TARGET_PROJECT_SYSTEM_INCLUDES is exported as a new product config
DeviceKernelHeaders.

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: I3f0a2b893cc9352d6c13f3151e8834477d15c07b