Add tests for the devicepolicy processor

Add unit tests for the new devicepolicy processor using
com.google.testing.compile.

Bug: 433951378
Flag: TEST_ONLY
Test: atest frameworks/base/tools/processors/devicepolicy/test/src/android/processor/devicepolicy/test/PolicyProcessorTest.kt
Change-Id: I94d5e62d1875131dbe94bf87e601c5726a8ac76a