Update mac_sdk_versions_supported.

Now we requires Mac OS X SDK version at least 10.8.
Also removed the unnecessary sort call in getting
mac_sdk_versions_installed.
Lexical sort doesn't make sense when 10.10 is added to that list.
Actually the next makefile line makes sure we always get the first
match in $(mac_sdk_versions_supported):
mac_sdk_version := $(firstword $(filter $(mac_sdk_versions_installed),
$(mac_sdk_versions_supported)))

Bug: 22825165
Change-Id: I4bfafabb11f94b66704d4799fe61fe3f16429538