Disable warning triggered in Clang r271374

http://b/28149048
http://b/29823425

Disable -Wconstant-conversion that gets triggered in
native/imageproc/to_rgba.c.

Test: Tested build, boot and common usage for Arm, Arm64, x86, x86_64,
Mips images in AOSP and internal branch.

Change-Id: Ia8dbe49a1a8577599244642cbd2e3bb17ec1f83c
1 file changed