android_os_SELinux.cpp: Refactor GetSELabelHandle

Slightly refactor GetSELabelHandle to abstract out the handle
fetching code and atomic cache used to store the SELinux handle.
This change will make it easier, in a future commit, to add additional
backends.

This should be a no-op.

Test: compiles and boots
Change-Id: I4a75daf8910d5c01bbd6732756ba9007e08c5bcb