Remove mis-specified @GuardedBy("mMethodMap")

This is a safe no-op CL that removes mis-specified

  @GuardedBy("mMethodMap")

annotations from InputMethodManagerService, which was added while we
were trying to introduce proto-based event logging [1].  Otherwise our
lint would keep false-alarming even if those methods were correctly
used.

Anyway, this CL should have zero impact on generated code since
@GuardedBy annotation is used only for static code analysis.

 [1]: Icbfb8c11e882f29eb45dea9d4c23315c48e9d619
      5e68eeac

Bug: 154348613
Bug: 177462676
Test: presubmit
Change-Id: I92fb0f9f8957ad0649bfad6196356bb3f9698e04