[HostStubGen] Tweak "filter reason" handling on...

... package policies.

Previously, when package policies always took precedence over
annotation policies (because we want the policy file to take
precedence over annotations). Even when a class already has a
@Keep annotation, if you also add a "keep" package policy,
the "reason" would be "file-override".

This is inconvenient when analyzing method call log. When a class
has an annotation, we want to know that.
(context: I934f26e38b4c965fad98ab4103b0d2baba70e238)

With this CL, now we:
- Package policies _still_ take precedence over annotations.
- But if the package policy and the annotation indicate the same policy,
  then we use the annotation as the filter reason.

(Another potential solution this problem is to analyze the @RavenwoodXxx
annotations as well, in addition to the @HostStubGenXxx annotations,
in RavenwoodMethodCallLogger, but that'd be kind of more hacky for
various reasons.)

Bug: 292141694
Flag: TEST_ONLY
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -r
Change-Id: I55a7aa614749c7d38725d7de484817af22ff185d