Add Plugin interface for Toasts

Allows Toasts that route through SystemUI to be implemented by a
ToastPlugin. This CL also adds the ability for plugins to create a
custom animation for when the toast shows and hides.

Also adds logging for Toasts that get routed through SystemUI. By
default, these logs aren't logged to logcat but can be enabled via adb (see
LogBuffer.kt).

To dump ToastLog:
adb shell dumpsys activity service com.android.systemui/.SystemUIService ToastLog

Bug: 169587378
Test: manually add CustomToastPlugin
Test: atest ToastUITest
Change-Id: I0a0b16fdc2a5ba1908054197f6dc6728f10a0d2e