Clear the Ember NCP key table when reforming the network

[cdjackson]

Clear all keys from the Ember key table when reforming the network. Without this, APS link keys may remain that will prevent devices from joining the new network when uses the trust centre link key.
Signed-off-by: Chris Jackson chris@cd-jackson.com

[codecov]

Codecov Report

Merging #700 into master will increase coverage by 0.01%.
The diff coverage is 87.5%.

@@             Coverage Diff              @@
##             master     #700      +/-   ##
============================================
+ Coverage     27.85%   27.87%   +0.01%     
- Complexity     3532     3534       +2     
============================================
  Files          1365     1365              
  Lines         48986    48994       +8     
  Branches       2114     2114              
============================================
+ Hits          13644    13655      +11     
+ Misses        34718    34715       -3     
  Partials        624      624

Impacted Files	Coverage Δ	Complexity Δ
...gle/ember/internal/EmberNetworkInitialisation.java	3.33% <0%> (-0.03%)	1 <0> (ø)
...om/zsmartsystems/zigbee/dongle/ember/EmberNcp.java	26.64% <100%> (+1.64%)	13 <1> (+1)	⬆️
...e/ember/ezsp/command/EzspClearKeyTableRequest.java	57.14% <0%> (+57.14%)	1% <0%> (+1%)	⬆️

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