[SettingsProvider] always destroy old tracker before inserting a new one

Previously, the destroying of the old tracker and the insertion of the
new tracker are in two separate synchronized blocks. This creates the
opportunity of a race condition, where another tracker is inserted
between the two synchronized blocks, and is never properly destroyed,
which leads to memory leaks. This CL fixes that.

BUG: 277448672
Test: builds; this is a theoretical fix as I couldn't reproduce the bug
locally

Change-Id: Ia0fbcee6f56dd1035e68d20f3730b4d34e2085b6