Build: Update Mips64 generic build for ART

For ART testing, we need:

1) A larger userdata partition. A lot of files end up there as it
is multi-arch.

2) Don't strip prebuilts. Technically we only care about core-libart,
but this is the best high-level change that doesn't impact other
files.

Change-Id: Ic36bfcf80ba50a602752ca0a3031dda89a0f3051