zram: support zram size experiments

Let vendor_init adjusted the zram size by device config from the
server. Allow us to study the performance and power impacts
by experiment settings.

Change persist.vendor.zram.size to persist.vendor.boot.zram.size since
it only takes effect when next booting.

Bug: 240128114
Test: adb remount ; adb push pixel-mm-gki.rc /vendor/etc/init/ ; adb reboot
Test: adb shell setprop persist.device_config.vendor_system_native_boot.zram_size 4g ;
adb reboot ; check zram swap size is 4g
Test: adb shell setprop persist.device_config.vendor_system_native_boot.zram_size 4g ;
adb shell setprop persist.vendor.boot.zram.size 3g ; adb reboot ;
check check zram swap size is 3g

Change-Id: I0bf62fe1275a1e0bed129d78db9b4e1e5c353dbe