Optimize calls to IME's onStartInput

Optimize calls to IME's onStartInput

As of today onStartInput is called for every window which gains focus. We
can significantly optimize this by restricting calls only to windows
that have a text editor. However, if input was already started based on
window flags, behavior is unchanged.

This behavior is guarded by debug flag "debug.optimize_startinput" and
is defaulted to true.

Bug: 37617707
Test: atest CtsInputMethodTestCases
Test: Verify following command reverts the behavior: adb root && adb
      shell setprop debug.optimize_startinput false &&
      adb shell cmd input_method refresh_debug_options
Change-Id: Ibf9dab3d9c138b5f04e053d41ee4fd248c78e4da