Set BOARD_SYSTEMSDK_VERSIONS automatically for P-launching device

Like BOARD_VNDK_VERSION is automatically set, BOARD_SYSTEMSDK_VERSIONS
is also automatically set to the latest supported SystemSDK version when
it is unset. Specifically, it is set when

1) it isn't explicitly set by the vendor and
2) the device is P-launching device (PRODUCT_SHIPPING_API_LEVEL >= 28)

Bug: 78431725
Test: m -j
Test: `get_build_var BOARD_SYSTEMSDK_VERSIONS` return P for 2018 Pixels.
Change-Id: I036a07da3ce58bdf22d252b6ef18263c7da3602a