perfmgr: Add helpers to interact with GPU driver.

Adds the rigging to write to the GPU capacity node, if this exists.
This might be better as an ioctl at some point, but for now use the
existing interfaces which are also guarded by sepolicy for access
control.

Bug: 290625326
Test: new unit test for GpuCapacityNode.

Change-Id: I32c0790996ccf7322d63a3beb12ca56411b9b4d4
4 files changed