surfaceflinger: Replace blank/unblank with setPowerMode

We replace the blank/unblank calls in surfaceFlinger with a more generic
setPowerMode() routine.

Some displays support different power modes (for example, with reduced
color palettes). Depending on the use case we should be able to toggle
these modes, so as to achieve incremental power savings.

Initially, three power modes will be supported:
- HWC_POWER_MODE_OFF
- HWC_POWER_MODE_DOZE
- HWC_POWER_MODE_NORMAL

HWC_POWER_MODE_OFF will correspond to blanking the display, while
HWC_POWER_MODE_NORMAL will correspond to unblanking. HWC_POWER_MODE_DOZE
will put the display into a low power setting, if it is supported in
hardware.

If such a low power mode is not supported, it should be treated as a
call to set the mode to HWC_POWER_MODE_NORMAL.

As a consequence of adding the mPowerMode field, the mScreenAcquired is
no longer required, and thus references to it are removed and replaced
equivalent references to mPowerMode.

We also add the glue code to connect the services invocation of setting
a power mode and the HAL implementation in HWComposer.

Bug: 13472578
Change-Id: I431595ecf16d2f2c94259272db3dd42f29636204
Signed-off-by: Prashant Malani <pmalani@google.com>