Explicitly specify thermal throttling type

Currently display code supports only one type of throttling: thermal-based. And
so today many places and identifiers implicitly mean thermal throttling. We are
going to introduce more types of throttling soon (battery-level based, power
based), so this CL renames a number of fields and structs to explicitly include
the word 'thermal', to make it easier to differentiate and reason about
different types of throttling in the future.

Test: build an OTA
Test: atest BrightnessThrottlerTest DeviceStateToLayoutMapTest DisplayDeviceConfigTest LogicalDisplayMapperTest DisplayPowerControllerTest DisplayPowerController2Test
Bug: 262458486
Change-Id: I93d3033c049e17355dd850f4e7e3f2a64c361e57