Refactor GSI BoardConfig.mk

The patch modifies the Legacy GSI to use BoardConfigGsiCommon.mk
instead of board config treble-common*.mk, and delete these
treble-common*.mk files.

After the patch, the GSI common board configs are in single file
BoardConfigGsiCommon.mk

The patch also includes:

- Moving `TARGET_COPY_OUT_VENDOR := vendor` to
  BoardConfigGsiCommon.mk, because GSI always contains no vendor
  contents.
- Configure the binder interface bitness explicitly.
- Unify the structure of each BoardConfig.mk.

Bug: 122282239
Test: compare the out folder between with and without the patch
Test: output files are all the same
Change-Id: Ie5901446920dda317dea848e6d5a7bbcf10a32cf