Fix failure to load power HAL implementation

Some working implementations of the libhardware power HAL do not
implement an open method. Change the HIDL implementation to not rely
on that method being available.

Test: Flashed marlin with modified power.marlin that omits an open()
implementation; power HAL loads successfully and passes VTS
Bug: 37245218
Signed-off-by: Connor O'Brien <connoro@google.com>

Change-Id: Icc77b3011a0e44330062aa51ae49e39ad349a2a5