Define ro.board.api_level property

ro.board.api_level shows the current vendor api level under GRF.
It can be manually defined by setting BOARD_API_LEVEL. Unless
BOARD_API_LEVEL is defined, the ro.board.api_level property will be
defined automatically based on BOARD_SHIPPING_API_LEVEL and
PLATFORM_SDK_VERSION.

Bug: 176950752
Test: getprop ro.board.api_level
Change-Id: I03eeec8d8206abdd0565423d1b6a507d86d9b168
diff --git a/private/property_contexts b/private/property_contexts
index 6a00538..20904ee 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -824,6 +824,7 @@
 
 # GRF property for the first api level of the vendor partition
 ro.board.first_api_level u:object_r:build_vendor_prop:s0 exact int
+ro.board.api_level       u:object_r:build_vendor_prop:s0 exact int
 
 # Boot image build props set by /{second_stage_resources/,}boot/etc/build.prop
 ro.bootimage.build.date                        u:object_r:build_bootimage_prop:s0 exact string