Split up annotation processing

Split up annotation processing into @PolicyDefinition,
@EnumPolicyDefinition and @BooleanPolicyDefinition handling. Also
extensively documents what each processor does.

Bug: 442556276
Test: atest frameworks/base/tools/processors/devicepolicy/test/src/android/processor/devicepolicy/test/PolicyProcessorTest.kt
Flag: android.app.admin.flags.policy_streamlining
Change-Id: I7aa85d8b5c35244664a2e45666e35f16ecc88bce