Use a single lock and add a flag.

Using separate locks adds a little bit of overhead to the get method.
Switch to a single lock to minimize the performance cost.

Also add a control flag to disable caching in the future, if so desired.

Bug: 257054989
Test: atest CorePerfTests:AccountManagerPerfTest
Change-Id: I5bbcc54f28cbe3c434995af8b3452debfc68c4ee