Support for API exemptions from API blacklisting.

Add support to VMRuntime for setting a list of exemptions from the hidden
API enforcement. The list is used as a prefix match against the signature
of a field/method. Any signatures that match are treated as being on the
light grey list, rather than the dark grey list or blacklist.

Refactor some code in hidden_api.h that deals with field/method signatures,
to encapsulate the signature in a new class MemberSignature. This allows us
to avoid building the entire signature in member, instead just using its
constituent parts.

Test: $ make test-art-host-gtest-hidden_api_test
Test: $ adb shell settings put global hidden_api_blacklist_exemptions \
Test:    Landroid/view/RemoteAnimationDefinition\\\;:Landroid/app/ActivityManager\\\$TaskDescription\\\;
Test: Manually verify logcat output from app which uses named APIs
Bug: 73337509
Change-Id: Id608743d1b5a7a37059875d8991d0d4d65f5fc36