screen_ui: Call gr_exit() from dtor.

ScreenRecoveryUI calls gr_init() during Init(), and should release any
allocated resource upon destructing. It's a no-op to call gr_exit() if
gr_init() was not called or didn't finish successfully.

Test: recovery_unit_test on marlin.
Test: Boot into recovery. Run graphics test.
Change-Id: Ib9632170174cdacd51d382f23c8daf6a9bc59f7a

screen_ui.cpp

@@ -176,6 +176,8 @@ ScreenRecoveryUI::~ScreenRecoveryUI() {
   if (progress_thread_.joinable()) {
     progress_thread_.join();
   }
+  // No-op if gr_init() (via Init()) was not called or had failed.
+  gr_exit();
 }
 
 GRSurface* ScreenRecoveryUI::GetCurrentFrame() const {