Gallery2 : only build for 32-bit

Gallery2 depends on librsjni, which currently only builds for arm,
mips and x86.

(cherry picked from commit c9f714b65cdb05cef4797f30a42d006b80140aaf)

Change-Id: I7fd5363b453ca149bbd24e889f03a7bae6f4032e
Merge "Gallery2 : only build for 32-bit"
auto-merger: merge in from aosp/master