Allow to strip everything for only some build variants.

When LOCAL_STRIP_MODULE is keep_symbols, you can still use
STRIP_EVERYTHING_BUILD_VARIANTS in product configuration to strip
everything for some build variants, such as user build to save image
space.

Bug: 16897368
Change-Id: I2a1b7204e5c976387ddea8846c82e11a7b478d8d