Restrict access to private flags.

Specifially restrict access to mPrivateFlags and mPrivateFlags2. We
were not able to find a legitimate use-case. Several flags have
corresponding public APIs. We are deliberately breaking the usage
of any flags that don't have alternative API already as it breaks
encapsulation of the View class that is supposed to manage these
flags on its own.

Some examples of existing public APIs for important flags:
PFLAG_FOCUSED
- hasFocus

PFLAG_SELECTED
- isSelected
- setSelected

PFLAG_HOVERED
- isHovered
- setHovered

PFLAG2_ACCESSIBILITY_FOCUSED
- isAccessibilityFocused
- requestAccessibilityFocus

Bug: b/123769414
Bug: b/123768943
Bug: b/129147060
Test: N/A
Change-Id: If8acd7e8b516e4ce6808dcd57485659b616e24ee