vold: skip first disk change when converting MBR to GPT

When converting a public (MBR) partition to private (GPT) partition,
'sgdisk --zap-all <path>' triggers a disk change netlink event
when converting from MBR to GPT.  Then, 'sgdisk --new=....' triggers
another disk change netlink event.

vold informs clients a new volume is created after the first disk
change event occurs.  system server reacts by requesting to mount
the volume.  If this request is honored before the second disk change
event, the volume will be unmounted immediately after system server's
request to mount is honored.  The next time system server performs
an operation (createnewuser) on this volume, it will fail due to
the volume being unmounted.

This is reproduced by running the following commands in a loop:

adb shell sm partition <disk> private
adb shell sm partition <disk> public
adb shell sm forget all


run cts -c com.android.cts.appsecurity.AdoptableHostTest -m testPackageInstaller

This change causes vold to delay notifying clients that the volume is
ready until after it's actually partitioned.

Change-Id: I457cc1508573d73ef2be2f0cfdc5c2237bfabad7
2 files changed