chore(#Magnification): add MagnificationFeatureFlagBase

We'll add thumbnail feature flag in next cl and joystick feature flag in the future. These feature flag files are almost same as AlwaysOnMagnificationFeatureFlag, just in difference on namespace and feature name. Therefore, we add a feature flag base file to handle the logics, so that the extended feature flag files can share the base codes and customize namespace and feature name on its own.

In MagnificationFeatureFlagBase, we would clear binder identity, then access the DEVICE_CONFIG with given namespace. It's to prevent permission denied exceptions. we would restore the identity after the access.

Bug: 276979625
Test: manually
      atest MagnificationControllerTest
Change-Id: I2857cd332bd98726718cf5b5913e6eccf741c3da