Add Context.getSystemService(Class<?>).

Added an overload of getSystemService() that takes a class instead of a
service name to eliminate the extra cast and provide more type safety.

Cleaned up docs.

Removed the default constructor of BatteryManager which should not have
existed.

Change-Id: I9da46b20641fc83ecd3342560e5b94cb721f289c
11 files changed