TokenWatcher: fix bug in repeated calls to acquire().

The javadoc of TokenWatcher promised to implement repeated calls to
acquire() with the same token as a no-op, but this wasn't the case.

When acquire() is called repeatedly with the same token, its associated
DeathRecipient object is replaced by a new one. The old DeathRecipient
can therefore be destroyed. This is problematic because the finalizer of
this DeathRecipient implementation calls release() on the same token,
resulting in the token being released unexpectedly.

In this CL we properly implement the documented no-op behavior.

Bug: 68368071
Test: (First, cherry-pick to internal master)
Test: cts-tradefed run cts-dev -m OsTest -t android.os.cts.TokenWatcherTest
Test: CTS verifier > Managed provisioning > Device owner tests
      > LockTask UI

Change-Id: Idfad7c88075a510ea1f8527b5a8620121a5dd7bb