property_contexts: strip blank lines and comments

Strip whitespace and comments from property_context files
to reduce size. On an aosp_x86_64 build it saves 851 bytes.

However, on builds with more synclines and comments, further
space savings can be realized.

Change-Id: I43caf1deaab53d4753c835918898c8982f477ef0
Signed-off-by: William Roberts <william.c.roberts@intel.com>