Rename INSTALLED_SYSTEMIMAGE to INSTALLED_SYSTEMIMAGE_TARGET.

This gives a consistent mapping between partition names and the image
targets (i.e. partition => INSTALLED_PARTITIONIMAGE_TARGET). This CL
also creates an alias for the old name so that device-specific Makefiles
that refer to INSTALLED_SYSTEMIMAGE would keep working.

This CL renames image-for-partitions to images-for-partitions, and
returns INSTALLED_SYSTEMIMAGE_TARGET for system. It doesn't change the
actual functionality, but allows substantially simplifying the rule.

Test: `m dist` with aosp_taimen-userdebug
Test: `m dist` with a target that uses super partition
Change-Id: I2954ee5d855c1652a33c640ee615649588d5bc04