Update View docs for pointer icon and default resolution behavior

We now support showing a pointer icon for styluses in Android. When
a device is configured to show pointer icons for styluses, apps can
select the pointer icon to be used for the stylus pointer by overriding
onResolvePointerIcon.

Previously, onResolvePointerIcon was only called for mouse events, but
now it's possible that it will be called for stylus pointers as well.
To limit unexpected behavior in existing apps that only expect pointer
icons to be shown for mouse devices, a pointer icon set using
View#setPointerIcon will now only apply to mouse pointers. We update
the docs to reflect these changes, and add instructions for explicitly
resolving stylus pointers.

Bug: 215436642
Test: atest PointerIconTest
Change-Id: I40364cb01277cb9c98704dfc1cbee9dbfd526000