Android: enable writing of characteristics without response mode

Change-Id: I9c26aaa11857db8dc33a99d42347a9b7f6281ad7
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Don't update cached char value when property is not readable

In addition we update the documentation to reflect the slightly changed
API behavior.

Change-Id: Ieddee750aa35a32d3c01213dfbf678ee2a1d88d7
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Low energy descriptor - value on iOS

On OS X CBDescriptor's value is always NSData,
on iOS it can be NSData, NSString, NSNumber.

Change-Id: I6ffa4b813238ec26759508a012fa6e236db04c63
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>