Always use Context.getSystemService() to get IMM

This is a preparation to deprecate the following two methods.

 * InputMethodManager#getInstance()
 * InputMethodManager#peekInstance()

Since we soon need to stop relying on the current per-process
InputMethodManager singleton model to fully support multi-display,
those two methods really need to be deprecated.

With this CL, framework code no longer depends on
InputMethodManager#peekInstance(), which is also marked as deprecated
in this CL.

InputMethodManager#getInstance() is a bit tricky because it also works
as a constructor of such a per-process singleton instance.  Remaining
two call-sites of this method will be migrated in a subsequent CL.

This is a mechanical refactoring, which in theory should have no
observable logical behavior difference.

There could be a small performance regression, but it is basically not
avoidable to correctly support multi-display scenarios.

Bug: 115891476
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest CtsWidgetTestCases:android.widget.cts.EditTextTest
Test: atest CtsWidgetTestCases:android.widget.cts.TextViewTest
Test: atest FrameworksCoreTests:com.android.internal.inputmethod
Test: atest FrameworksServicesTests:com.android.server.textservices
Change-Id: I5db31491f3d47d3ad4a621e956995135c72e007b