Combine UsbManager.setPrimaryFunction and setDefaultFunction

Due to the property trigger on persist.sys.usb.config,
setting the default function also sets the current function.
Now we combine both of these methods into setCurrentFunction, which has
a "makeDefault" option to make the new function the default.

This change should eliminate some problems with setting properties due to
multiple property triggers happening at the same time.

Change-Id: I9851299e9c2ee20475eada1a8104c0d50bf5a9e1
Signed-off-by: Mike Lockwood <lockwood@android.com>