Remove unnecessary power manager wrapper for wmtests

Because SystemServicesTestRule uses:
 final PowerManager pm = mock(PowerManager.class);
 doReturn(pm).when(mContext).getSystemService(eq(POWER_SERVICE));
The methods of PowerManager are already no-op and verifiable.

Bug: 163976519
Test: WindowStateTests#testPrepareWindowToDisplayDuringRelayout
Change-Id: I5d6396a0d0975c89de1d17fdb884a676b8d359cc