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.
4. Moved some static variables into non-static.

Bug: 239607619
Test: atest TelephonyFrameworksTests
Change-Id: I50c110f53caec4b73708dcf1cb8d52708242d7ea