Test frozen sepolicy has not diverged from prebuilts.

This will test that system/sepolicy/{public/, private/} are identical to
prebuilts if PLATFORM_SEPOLICY_VERSION is not 10000.0.

Bug: 74622750
Test: build policy
Test: correctly catches divergence from prebuilts for frozen policies

Change-Id: I2fa14b672544a021c2d42ad5968dfbac21b72f6a