[MasterClear] fix race in showing dialog

Wiping the PDB effectively starves all other threads from
accessing flash, and freezes the UI. We throw up a dialog
but there's a race between drawing the dialog and starting the
wipe.

Show the dialog in onPreExecute to fix this.

Bug: 20854355
Change-Id: I8462f5ed3ff53ed574ebc1be416a821c2086b4f5