Introduce a custom intent action to close software keyboard

This CL introduces a custom intent action for apps to ask AOSP Keyboard to
close its software keyboard with guarding it with a signature-protected
permission.

Any app that is signed with the same signature as AOSP Keyboard can have
the following line in AndroidManifest.xml

  <uses-permission
          android:name="com.android.inputmethod.latin.HIDE_SOFT_INPUT"/>

to request AOSP Keyboard to close its software keyboard as follows.

  sendBroadcast(new Intent("com.android.inputmethod.latin.HIDE_SOFT_INPUT")
          .setPackage("com.android.inputmethod.latin"));

Test: Manually verified with a test app.
Fixes: 65270710
Change-Id: I4fd2e3a7336ec66c70582a2f274a200cbf035a7f