fix(PointAnnotationManager): override getDelegate on Android view manager (#4246)

* fix(PointAnnotationManager): override getDelegate on Android view manager

RNMBXPointAnnotationManagerViewManager (added in #4240) never overrode
getDelegate(), so under Fabric its props (slot, iconAllowOverlap, …) were
never applied and React Native logged:

  ReactNoCrashSoftException: ViewManager using codegen must override
  getDelegate method (name: RNMBXPointAnnotationManager).

As a result multiple PointAnnotationManagers rendered blank on Android.
Provide the codegen delegate like the other view managers
(RNMBXCalloutManager, RNMBXMarkerViewManager).

* fix(PointAnnotationManager): don't clobber Android iconAllowOverlap default

The Android annotation plugin defaults iconAllowOverlap/iconIgnorePlacement
to true so annotations always show. applyProps() unconditionally wrote the
(nullable) props, so when they were unset it pushed null and reset the
manager to the style-spec default (false), culling colliding pins. In the
PointAnnotationManager Slot example this dropped pin 2. Only write each
option when the prop is actually set.