build: no longer build emulator by default

This is just increasing build times when most people are not using it.
Let's speed up builds a bit.

Building emulator should still be possible by appending BUILD_EMULATOR=true
to the build command

PS2: Let's keep the darwin warning.

Change-Id: Ibd8e2b90455bd603a691c7b95466068b6fae212a
Signed-off-by: Andrew Warfield <hiasant@gmail.com>
Always rebuild build.prop file

build.prop file was not rebuilt after changes to $(PRODUCT_PROPERTY_OVERRIDES)
Without this patch, if change I681fd92e will be merged,
the wrong date will be always in the zip file's name
(date of building build.prop and not the current date).
This patch simply forces rebuilding build.prop every time.

Change-Id: I5071072cdbb34865e32403a2e7ce0f705ff9daee