Calculate ro.vendor.api_level with the new vendor API format

In Android V, ro.vendor.api_level read ro.board.api_level only if
ro.board.first_api_level is defined. The format of the vendor api
level is YYYYMM.
Update the ro.vendor.api_level initialize funtion with this change.

As ro.product.first_api_level follows the version from the SDK, a
new function `vendor_api_level_of` translates the SDK version to the
vendor API level.

Bug: 312403948
Test: getprop ro.vendor.api_level
Change-Id: I2961ed9a6de8e9156f72c1bbfeabcac9c35f08fb