[neurun] Introduce ConstantInitializer (#3426)

[neurun] Introduce ConstantInitializer (#3426)

* [neurun] Introduce ConstantInitializer

This commit introduces `ConstantIntializer` class that fills operand
objects with the constant values which were specified by
`ANeuralNetworksModel_setOperandValue`. Plus, this class replaces the
InitializerGenerator.

NOTE due to FullyConnected operation's implicit reshaping, it is
implemented with workarounds.

Signed-off-by: Hanjoung Lee <hanjoung.lee@samsung.com>
* Add license

Signed-off-by: Hanjoung Lee <hanjoung.lee@samsung.com>