Add a test API for testing NetworkStack.getService

NetworkStack.getService was introduced to avoid relying on
Context#getSystemService to obtain the NetworkStack binder token.
To allow it to be mocked in tests, a method is introduced to allow tests
to specify a mock NetworkStack token for their own process.

Test: atest NetworkStackTests, using the change
Bug: 151243982
Change-Id: I04058a007f2dfe1044cabeb3ac508404873665ad