Add permissions check for brightness changes via shell.

Also
- Adjusts some code structure around DisplayManagerShellCommand.
- Remove clearIdentity() on other display shell commands.

Test: Run all shell options for display in adb root and unroot.
Test: Verify that 'adb shell cmd display set-brightness *' still works.
Test: atest DisplayTest
Bug: 144920149
Change-Id: I9c6f584537a89a99f38871e280796fe66feeb312