Allow secondary user backup to USB

[Uldiniad]

By default, Android exposes USB devices only to the main user.
In order to query, read and write to it, the signature permission INTERACT_ACROSS_USERS_FULL (optional) is granted to create Seedvault's context as the system user.

Change-Id: I0b1b4c8c5aeeb226419ff94e15f631ebe1db66df
Issue: calyxos#437
Issue: #77

[grote]

Looks good. I found some minor stuff and will do one last pass before merging, checking all the context usage in the places you touched to make sure we are not accidentally using the wrong context somewhere.

[Uldiniad]

02-16 16:35:59.761 12887 12990 E DocumentsStorage: Error finding file blocking
02-16 16:35:59.761 12887 12990 E DocumentsStorage: java.io.IOException
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt$getLoadedCursor$2.invokeSuspend(DocumentsStorage.kt:281)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt$getLoadedCursor$2.invoke(Unknown Source:8)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt$getLoadedCursor$2.invoke(Unknown Source:4)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturnIgnoreTimeout(Undispatched.kt:102)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.TimeoutKt.setupTimeout(Timeout.kt:78)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.TimeoutKt.access$setupTimeout(Timeout.kt:1)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.TimeoutKt.withTimeout(Timeout.kt:31)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt.getLoadedCursor(DocumentsStorage.kt:279)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt.getLoadedCursor$default(DocumentsStorage.kt:278)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt.listFilesBlocking(DocumentsStorage.kt:204)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsStorageKt.findFileBlocking(DocumentsStorage.kt:248)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.plugins.saf.DocumentsProviderStoragePlugin.hasBackup(DocumentsProviderStoragePlugin.kt:82)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at com.stevesoltys.seedvault.ui.storage.RestoreStorageViewModel$onLocationSet$1.invokeSuspend(RestoreStorageViewModel.kt:28)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:241)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:594)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.scheduling.CoroutineScheduler.access$runSafely(CoroutineScheduler.kt:60)
02-16 16:35:59.761 12887 12990 E DocumentsStorage:      at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:740)

this is the error when save storage is not executed