libhidlbase: work w/ cfi

In a couple of places, in order to reduce templated code, HIDL relies on
auto-generated classes being exactly the same.

- one of these places, this reliance is unnecessary, so it's removed
- one of these places, explicitly documenting assumptions and turning
  off cfi

Fixes: 151006394
Test: hidl_test (w/ libhidlbase cfi on)
Test: atest MediaDrmClearkeyTest (no more cfi error)
Change-Id: I4834d38bcdaa2bb94e014f4c749251ee2025af15
2 files changed