Implement experimental RAM limiting for HSUM testing

Introduces `CONFIG_HSUM_EXPERIMENTAL_RAM_LIMIT` to dynamically restrict
device RAM via `BOARD_KERNEL_CMDLINE`. This enables efficiency testing
of HSUM on simulated low-memory configurations.

Test: Usage: `m CONFIG_HSUM_EXPERIMENTAL_RAM_LIMIT=4G -j16`
Verification: `adb shell "cat /proc/meminfo | grep MemTotal"` Output: MemTotal:        3536056 kB

Flag: EXEMPT experimental change only
Bug: 421895247

Change-Id: Ie8ee0e493ba798188f60f6d9406f92621dbb7cdc