Don't build without proprietary blobs.

Prevent broken builds of proprietary files are missing.

Change-Id: I8357476adbdc484e76c49cfa89fffae44bf1463c
Set TARGET_ARCH_LOWMEM to reduce Dalvik heap memory usage

See
https://github.com/CyanogenMod/android_dalvik/commit/15726c81059b74bf2352db29a3decfc4ea9c1428
for details.

Change-Id: I3623a0a0fd94df37806d14ceee42af95d15b2a90