healthd: move global gHealth service pointer

... to class implementation::Health. Expose APIs
InitInstance(), GetImplmentation()
for its setter and getter.

Clients that statically links to android.hardware
.health@2.0-impl (e.g. charger, recovery)
should use InitInstance().

Test: health VTS tests
Bug: 63702641
Change-Id: I51b9b976d0b723dac1a03e5113d5d9e84300c0e9