healthd: refactor to split charger and framework facing code

The charger code lands in 'libhealthd_charger' and androoid/framework
facing code is now compiled into 'libhealtdh_android' (formerly
libhealthd_internal).

This is a pre-cursor to eventually splitting healthd into 2 binaries,
viz ..

 1. /sbin/charger - statically linked binary to be used *only* in recovery
 and  charge-only mode(s).
 2. /system/bin/healthd - dynamically linked binary to be used *only* in
 Android.

The static board hal (i.e libhealthd.<target>) will then be compiled
twice for charger and healthd seperately, paving the way for us to
seperate out the HAL for Android

Test: Boot tested and verified 'healthd' works in recovery,
charger-only mode and Android.

Change-Id: If45213ddd14c00ea3b8e1759cfd99286ca1ef2ea
Signed-off-by: Sandeep Patil <sspatil@google.com>