Use BOARD_API_LEVEL to define ro.board.api_level

GRF devices must define the API level of which the SoC is first
shipped by setting BOARD_SHIPPING_API_LEVEL. As this is a permanent
value, vendors may not change this value even if they implement new
features under the GRF policy.

BOARD_API_LEVEL can be optionally defined in this case to manually
set the api level of the vendor implementation.
The current api level will be set to `ro.board.api_level` property.

Bug: 176950752
Test: atest --host post_process_props_unittest
Change-Id: Ib126c1a622ded9848650f3f60c0f15005867272d