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
(clean cherry-pick from internal branch)
Merged-In: I04058a007f2dfe1044cabeb3ac508404873665ad

Change-Id: I0e626d871176112a8575d629a05cb7a935b577a6