Reduce the system partition size of generic system images

Some device has small ROM size, so the GSI should be as small as
possible. The patch change the image sizes to be:

- 64 projects (default):    1.25GB (1280MB)
- 32 projects (default):    0.75GB  (768MB)
- ARM32 projects:                    734MB

Bug: 64458958
Test: Build all GSI projects
Change-Id: I7f7a266e48e99b0b4d5c6a84b8187b56e4482c94