use property_info to get security contexts

The file(/dev/__properties__/property_info) has already read the
property_contexts files, so use it instead of reading property_contexts
files.
Signed-off-by: liwugang <liwugang@163.com>

Support for Andriod R

Signed-off-by: liwugang <liwugang@163.com>

Support for Android Q

Signed-off-by: liwugang <liwugang@163.com>