Add minimal post processing flag in Settings.Secure

Minimal post processing might not be very well supported on all TV
displays (e.g.the switch to/out of the low latency mode might be too
slow or there might be flickering on the screen or some color
interference, etc). This depends on the vendor implementation of minimal
post processing. In such cases, we want to give the user an option to
disable this feature in the settings menu. This means that any calls to
the API (Window.setMinimalPostProcessing) will be ignored and no signals
will be sent to the display.

This CL adds the necessary flag to Settings.Secure. The value is read
from DisplayManagerService, which combines it with the information from
the WindowManager and the display capabilities to request the right low
latency mode on the display.

Bug: 144356279

Test: m && ./vendor/google/tools/flashall
      1. adb shell settings put secure minimal_post_processing_allowed 1
      2. Open Activity that prefers minimal post processing
      3. adb shell settings put secure minimal_post_processing_allowed 0
         -> this should turn ALLM off
      4. adb shell settings put secure minimal_post_processing_allowed 1
         -> this should turn ALLM back on
      5. Exit Activity
         -> this should turn ALLM off

Change-Id: I2d24a9a004d4939b0439d466ef2ca279803fd67f