locksettings: fix some errorprone warnings

Address all of the following classes of warnings:

- AndroidFrameworkBinderIdentity
- AndroidFrameworkUid
- ClassCanBeStatic
- EmptyBlockTag
- JUnitAmbiguousTestClass
- UnusedVariable

Also address some of the following class of warnings:

- AndroidFrameworkEfficientStrings

No "real" bugs were found or fixed; this is just a cleanup.

Test: atest com.android.server.locksettings
Test: m services FrameworksServicesTests RUN_ERROR_PRONE=true
Change-Id: Ife7da635803ee6de630f4949309c8c18f5d67973