docs: Removing most of the intro to PowerManager API

The intro text says PowerManager is discouraged, and almost
all the available wakelock options have long been deprecated.
Given that, I think it makes sense to remove most of that
intro and just point devs to FLAG_KEEP_SCREEN_ON instead.

Staged to:
go/dac-stage/reference/android/os/PowerManager

Test: make ds-docs
Bug: 145699347
Change-Id: I517366903f3d9743166d7edaddc08471af0803d9