Allow returned screenshot URI to be null

Occurs in failure cases (if the screenshot does not successfully save).

Based on ag/26920432 but couldn't cherrypick over directly because the
underlying code is different in this branch.

Flag: NONE
Test: atest com.android.systemui.screenshot com.google.android.systemui.screenshot
Bug: 329659738

Merged-In: I891eac48ccf77101dc5ff2fcd0dd492c53343e59
Change-Id: I017f64c577e046dd0ad522098ddf426c51df3c50