Set default size for 32-bit GSI to 1 GB

The previous size for 32-bit GSI (906 MB) is not enough for x86 GSI.
Increase it to 1 GB (1024 MB). If some other arch, say arm, wants to
use a smaller size, it can overwrite the default in its BoardConfig.mk.

Bug: 65437122
Test: lunch aosp_x86_a; make -j; lunch aosp_arm_a; make -j
Change-Id: I092966f0a0a327b1080ee10782960c6ccf44d8ee