[HostStubGen] Now method descriptor is optional in policy files

Originally, I wanted to avoid over-exposing unnecessary methods, so
I made this check strict.

However, policy files are basically for prototyping, and we should prefer
annotations anyway, so the strictness is just unnecessary.

Flag: EXEMPT host test change only
Bug: 292141694
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Change-Id: I0d7ec4e5f06f048842892c809a8a5646512f6616