utils: add method to check file paths and return the path of an existing file

this is useful as on different phones there may are different locations for
files which have the same function, eg torch.

this allows to pass in an array to the method, it checks the paths and if
a file exists, the path of it is getting returned.

also fixup the vibrator and bootreceiver:
  *) only do anything at bootup if we actually have started DC before
  *) vibrate after writing the value to the system, else it uses the old value
  *) remove a redundant preference saving call

Change-Id: Ia0852b266e7626be28175112e4d2ddc4b0226ba8