PermissionCache: expose a method to purge permission cache

Expose PermissionCache::purge() method. This is needed when running CTS
tests that adopt the shell permission identity to make sure the CTS apk
permissions are checked again after adopting the shell permission.

Bug: 185972521
Test: AudioRecord regression tests

Change-Id: I2c013eb857917e4c4477abf6237d0a76ef70aa9a
Merged-In: I2c013eb857917e4c4477abf6237d0a76ef70aa9a