Split GSI and mainline board configs

Make the mainline board config only import a subset of the GSI
options. The current set of attributes is the minimal set to
keep mainline_system_arm64 building, as well as not producing
any diffs in the current set of files in /system. It's possible
we'll need to move a few more over, but we'll do that on a
case-by-case basis.

Make the GSI config inherit the mainline one. The only diff for GSI
should be the addition of BOARD_USES_SYSTEM_OTHER_ODEX.

Bug: 119911662
Test: build mainline_system_arm64
Change-Id: Id5d114bde8b93800fe52c791ccc47cdffd86f382