Reduce the calling times of getCustomizedContextOrDefault()

In the current design, the getCustomizedContextOrDefault() will
be called for 6 times indirectly in the constructor of
NetworkMonitor.
Have a mCustomizedContext and get the value by calling
getCustomizedContextOrDefault() for one time to improve the
design.

Bug: 159093739
Test: 1. atest NetworkStackTests:NetworkMonitorTest
      2. atest NetworkStackNextTests:NetworkMonitorTest
Change-Id: Ie87df3130afc50a000af58bc638f04a008c4fa3d