Show a toast when a screenshot is taken and power saver is on.

Improvements to the power save policy for OC eliminated some
errant frames of the full animation when a screenshot is taken.
However, this resulted in no on-screen indication, which the toast
addresses.

Test: manual
Change-Id: I5bf0b534e9e848f1bbfdefec2ac637e39b7b75e4
Fixes: 37775089