sensor: Fix samples that assume incorrect sensor_value type.

Various sensor samples are hardwired to expect returned sensor values
are represented as doubles.  In each case this assumption is incorrect.

Introduce a generic sensor_value to double helper function and adjust
the samples to use it.

Change-Id: I89c788686576562b84e07a36064640231340c33b
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>