Add PowerManager#getThermalHeadroom

Adds getThermalHeadroom, which allows users to determine how close the
device currently is to SEVERE thermal throttling. For intensive
applications which can scale their workloads, this allows them to help
manage the thermal envelope of the device.

Test: Manual, modify Filament test app to call and observe values
Test: PowerManagerTest#testGetThermalHeadroom
Test: atest android.os.cts.PowerManager_ThermalTest
Test: atest com.android.systemui.statusbar.phone.StatusBarTest
Test: atest CachedDeviceStateServiceTest
Test: atest AbstractAccessibilityServiceConnectionTest
Test: atest KeyEventDispatcherTest
Test: atest AttentionManagerServiceTest
Test: atest HdmiControlServiceTest
Test: atest ThermalManagerServiceTest
Test: atest RecoverySystemServiceTest
Bug: 136285293
Change-Id: I26e1c849c4b08cbf7ee0057f555cf0893750da43