add searchpolicy.py for automated tests

searchpolicy.py provides a subset of the functionality of sesearch.

The primary benefit being that it's entirely built in-tree and thus
can be packaged for use in automated tests included compatibility
test suites.

Example
searchpolicy.py --libpath out/host/linux-x86/lib64/ --allow --source domain

Bug: 63397379
Test: Identical output with sesearch for the following commands
    --allow --source domain
    --allow --target domain
    --allow --target appdomain -p ioctl,open
    --allow --source lmkd -c file -p ioctl,open
    --allow --source lmkd -c file,dir -p ioctl,open
Change-Id: I89a6c333f1f519d9171fbc1aafe27eaf5ad247f0