Fix the missing lock in onPackageDataCleared()

As a preparation for supporting concurrent multi-user with
MyPackageMonitor, with this CL fixes a missing lock in

  MyPackageMonitor#onPackageDataCleared,

which was introduced recently [1].

As fields like

  InputMethodManager#mSettings
  InputMethodManager#mAdditionalSubtypeMap

need to be touched only within ImfLock, we still need an explicit lock
there.

 [1]: I159d9b7a2d1dcc8df478bb45d0be706d615724a2
      18a26c88

Bug: 267124364
Bug: 309837937
Bug: 322062773
Test: manually tested the case.
Change-Id: Ida9e9143324ccf0e5bcff224fbb0e4e6650996b0