We are compiling regex more than 300000 times, and it's a main
bottleneck for slow sepolicy_tests. Actually we don't need to compile
regex that much; most of cases can be handled by simple string
comparison. This change introduces heuristics for optimization.
Bug: 301874100
Test: verified that return values of MatchPathPrefix are not changed.
Test: run cProfile, before and after.
Before
ncalls tottime percall cumtime percall filename:lineno(function)
21951 0.923 0.000 56.491 0.003 policy.py:33(MatchPathPrefix)
After
ncalls tottime percall cumtime percall filename:lineno(function)
21951 0.078 0.000 1.159 0.000 policy.py:40(MatchPathPrefix)
Change-Id: I1ebad586c2518e74a8ca67024df5e77d068e3ca5
