Add call to set power mode for display

The blank/unblank interface is being replaced by a generic
setPowerMode() call. This will allow the support of low power modes in
displays where such functionality is available. Currently three modes
are defined:

- POWER_MODE_OFF
- POWER_MODE_DOZE
- POWER_MODE_NORMAL

POWER_MODE_OFF would be analogous to blanking the screen,
POWER_MODE_NORMAL akin to unblanking it, and POWER_MODE_DOZE would
trigger an entry into the display's low power mode.

We also tie the JNI call to set power mode to the call from services which
actually invokes it.

The generic setPowerMode() call can be expanded to potentially include
other display power states.

Bug: 13472578
Change-Id: I74677506d3ee2ccc50ba70c5102d96b31fe7b837
Signed-off-by: Prashant Malani <pmalani@google.com>