[RESTRICT AUTOMERGE] Ignore errors preparing user storage for existing users

Unfortunately we can't rule out the existence of devices where the user
storage wasn't properly prepared, due to StorageManagerService
previously ignoring errors from mVold.prepareUserStorage, combined with
OEMs potentially creating files in per-user directories too early.  And
forcing these broken devices to be factory reset upon taking an OTA is
not currently considered to be acceptable.

One option is to only check for prepareUserStorage errors on devices
that launched with T or later.  However, this is a serious issue and it
would be strongly preferable to do more than that.

Therefore, this CL makes it so that errors are checked for all new
users, rather than all new devices.  A field ignorePrepareStorageErrors
is added to the user record; it is only ever set to true implicitly,
when reading a user record from disk that lacks this field.  This field
is used by StorageManagerService to decide whether to check for errors.

Bug: 164488924
Bug: 224585613
Test: Intentionally made a device affected by this issue by reverting
      the CLs that introduced the error checks, and changing vold to
      inject an error into prepareUserStorage.   Then, flashed a build
      with this CL without wiping userdata.  The device still boots, as
      expected, and the log shows that the error was intentionally
      ignored.  Tested that if a second user is added, the error is
      *not* ignored and the second user's storage is destroyed before it
      can be used.  Finally, wiped the device and verified that it won't
      boot up anymore, as expected since error checking is enabled for
      the system user in that case.
Change-Id: I9bdd1a4bf5b14542adb901f264a91d489115c89b
(cherry picked from commit 60d8318c)
Merged-In: I9bdd1a4bf5b14542adb901f264a91d489115c89b