Strongly type the API lock.

Using an `Object` instead is slightly error-prone as it would allow
passing any type in e.g. constructor arguments (so one can easily end up
using any other `Object`).

Bug: 268611326
Test: atest CtsSafetyCenterTestCases
Change-Id: I7f4161bc062155609b52711e9c514bde97f5124a