health 1.0: add helper library for -impl

Now that libhealthd is no longer recognized as a HAL_STATIC_LIBRARY,
vendors must define their own android.hardware.health@1.0-impl module.
Add a helper and README.md for them to explain this transition.

Test: builds
Bug: 127677771
Change-Id: I65c162e7b5caed93c39a4a1cb6a2893fbb25724b