android: introduce Android.common.mk to reduce boilerplate

... across the makefiles. Currently this isn't much but that will change
shortly.

As an added bonus this fixes all present and future cases where we've
forgotten to strip out the headers from LOCAL_SRC_FILES.

In a couple of cases (the tests) we start setting
LOCAL_EXPORT_C_INCLUDE_DIRS, which shouldn't be an issue.

Cc: Chih-Wei Huang <cwhuang@android-x86.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>