Eliminate TaskViewModel's `mustRestartApp` field

Instead of having UserDataImportWarningDialog set an
`onResultDismiss` callback that examines `mustRestartApp`, and having
UserDataActivity set `mustRestartApp`, just have UserDataActivity set
the callback directly.

This approach is no more data-race-y than the previous approach, and it
simplifies the code. (The behavior of restarting the app when the task
finishes is specific to the user data import flow, and there is no
reason for TaskViewModel to be directly aware of it.)

Android: Replace deprecated setSystemUiVisibility

It happened due to a typo from SECTION_GFX_HARDWARE

Rename GetDefaultBackendName to GetDefaultBackendConfigName to
distinguish it from the display name.

GraphicsSettings: EFBAccessEnable=false by default

Makes Graphics -> Hacks -> Skip EFB Access from CPU enabled by default. Some GPU drivers stall when EFB access occurs in games where EFB is not used. Most games that require this setting set to 'true' already have this defined in their game inis.

Accessible from DolphinQt and Android.