Permissions for /dev/uinput

Currently /dev/uinput is owned by system/bluetooth.
But that's inconsistent with some of the sepolicies for uhid_device.
This also means that the new native tests for inputflinger aren't able
to execute properly, because they require the ability to register a new
input device via uinput.

Bug: none
Test: atest inputflinger_test
The newly added EventHub_test is still under review

Change-Id: I53524738db1a5d3ba962b9bec35ef322ed3028f2