Simplify IMM#hasActiveConnectionInternal(View)

After CL[1] and CL[2], IMM#hasActiveConnectionInternal(View)
only used by onPostWindowGainedFocus(View, LayoutParams).

However, the condition only need to verify the given view is
associated with current active input connection.

Hence we can remove the mServedInputConnection.isActive() check
to simplify the method.

[1]: I64412e87c4881018ea67140f9472e10ea41836e4
[2]: I7fe8b18ffbb450fd1e272e15f06f2629518f320c

Bug: 182259171
Test: atest CtsInputMethodTestCases
Test: make -j CtsInputMethod1  \
              CtsInputMethod2  \
              CtsInputMethodServiceDeviceTests  \
              CtsInputMethodServiceEventProvider  \
              CtsInputMethodServiceHostTestCases  \
              EditTextApp  &&
      atest -it  \
      CtsInputMethodServiceHostTestCases:InputMethodServiceLifecycleTest
Change-Id: Ib0c6827c58dd082a5f70b6ec508536b24cf42dcd