Subscription Manager public method clean up

1. Preliminary step of subscription manager API surface clean up.
2. Simpler way to solve unit test issue
3. Fixed incorrect string comparison.

Bug: 239607619
Test: atest TelephonyFrameworksTests
Merged-In: Ibf8907efd9dfcae5155c1b7b85a1411b61a13a77
Change-Id: Ibf8907efd9dfcae5155c1b7b85a1411b61a13a77