a11y: Fix typing while autoclick enabled

Because the autoclick panel is TYPE_APPLICATION_OVERLAY, when it was enabled it
permanently held focus which prevented typing and probably caused other issues.
Setting FLAG_NOT_FOCUSABLE resolves this issue.

Demo: http://b/399872186#comment2

Bug: b/399872186
Test: AutoclickTypePanelTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: If1f1ba54ce14a822a2177e8f891dc595c1945b0d