fastbootd: Remove all scratch partitions on update-super.

This ensures that the dynamic "scratch" partition is removed when doing
a flashall operation.

If "scratch" is on /data, disable the partition. Add IsImageDisabled to
ImageManager so EnsureScratchMapped can skip mapping it. Also, fix
"scratch" not getting unmapped if on /data.

Bug: 205987817
Test: adb remount
      adb sync
      adb reboot fastboot
      fastboot flashall --skip-reboot # no errors
Test: adb-remount-test.sh
Change-Id: I4b9702e1dac15fb663635506fb50a8274e1e10d1
diff --git a/fastboot/device/flashing.cpp b/fastboot/device/flashing.cpp
index 44dc81f..22f8363 100644
--- a/fastboot/device/flashing.cpp
+++ b/fastboot/device/flashing.cpp
@@ -186,6 +186,11 @@
     return result;
 }
 
+static void RemoveScratchPartition() {
+    AutoMountMetadata mount_metadata;
+    android::fs_mgr::TeardownAllOverlayForMountPoint();
+}
+
 bool UpdateSuper(FastbootDevice* device, const std::string& super_name, bool wipe) {
     std::vector<char> data = std::move(device->download_data());
     if (data.empty()) {
@@ -218,7 +223,7 @@
         if (!FlashPartitionTable(super_name, *new_metadata.get())) {
             return device->WriteFail("Unable to flash new partition table");
         }
-        android::fs_mgr::TeardownAllOverlayForMountPoint();
+        RemoveScratchPartition();
         sync();
         return device->WriteOkay("Successfully flashed partition table");
     }
@@ -262,7 +267,7 @@
     if (!UpdateAllPartitionMetadata(device, super_name, *new_metadata.get())) {
         return device->WriteFail("Unable to write new partition table");
     }
-    android::fs_mgr::TeardownAllOverlayForMountPoint();
+    RemoveScratchPartition();
     sync();
     return device->WriteOkay("Successfully updated partition table");
 }