libs: binder: MemoryHeapIon: ifdef ION_EXYNOS_VIDEO_MASK

Exynos secure memory regions were separated here:
https://android.googlesource.com/platform/hardware/samsung_slsi/exynos5/+/ce73ba1%5E!/

Only compile the legacy ION_EXYNOS_VIDEO_MASK code if legacy HALs are used
(and ION_EXYNOS_VIDEO_MASK is defined).

Change-Id: I8cb58bbbb829d64a2ed39dbd27cc0d0ae3c9dd09