healthd: allow devices to provide their own libhealthd

  * to use, set TARGET_PROVIDES_LIBHEALTHD := true

Change-Id: I5de702219fce6c0feb94a4c74978d81a1a12ca35
Add back DurationTimer to fix camera.msm8960 load

cannot locate symbol "_ZN7android13DurationTimer5startEv"

Change-Id: I29d3283b644c773e6d798d17b2ef139a470048da