fs_mgr follows system property 'ro.zram_backing_device_min_free_mb'
to check available space before creating zram backing device.
Previously, in Repair Mode, it requires at least 2 GB to boot
successfully with 512 MB zram backing device size. So we can set
1.5 GB as the minimum threshold.
Bug: 322821637
Bug: 326877642
Test: Fill up userdata, enlarge zram backing device size on next
boot.
Test: Boot to Repair Mode
Flag: EXEMPT bugfix
Change-Id: I16c6f1649181af94347b4534c790522bfd2fc166