Add accessibility event when saving screenshot

Send an accessibility event with "Saving screenshot" as the
screenshot is saved, so that it can be read out through e.g.
TalkBack.

Bug: 149832748
Test: manual; turned on talkback and ensured that the text was
read out when saving the screenshot from keychord/overview.

Change-Id: Ibe237a81cec0961065bf741eb5f3bb9dc9d14712