Handle the AM error dialogs in system UI thread

And don't hold the proclock while iteracting with the dialogs.

Bug: 186808162
Test: atest CtsWindowManagerDeviceTestCases
Test: atest CtsOsTestCases:StrictModeTest
Test: Manual - trigger ANR & kill app from adb shell
Change-Id: Ia7c16904fd734d122ee3459eeb579e9a78637204