Make sure IME token verification is done inside a lock

This caller verification needs to be done in an atomic manner.  There
is a possible race condition in the following code.

  @BinderThread
  public boolean doSomething(IBinder imeToken, ...) {
      if (!calledWithValidToken(imeToken)) {
          return false;
      }
      // possible race condition here.
      synchronized(mMethodMap) {
          // actual operations
      }
  }

Insted, we should check the IME token after taking a lock.

  @BinderThread
  public boolean doSomething(IBinder imeToken, ...) {
      synchronized(mMethodMap) {
          if (!calledWithValidTokenLocked(imeToken)) {
              return false;
          }

          // actual operations
      }
  }

Bug: 34886274
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Ia128b27de2cf16565c9c3fd40c5ac3be8e4eac42