Add unit tests to ActivityStarterInterceptor

Add some unit tests to the interceptor and also slightly change its
behaviour to return whether interception occurred to make the code in
ActivityStarter slightly more readable.

Test: bit
FrameworksServicesTests:com.android.server.am.ActivityStartInterceptorTest
Test: go/wm-smoke
Change-Id: I388727f2bbd96754ba67f9c777233adb46ede685