Prevent exfiltration of system files via avatar picker.

This adds mitigations to prevent system files being exfiltrated
via the settings content provider when a content URI is provided
as a chosen user image.

The mitigations are:

1) Copy the image to a new URI rather than the existing takePictureUri
prior to cropping.

2) Only allow a system handler to respond to the CROP intent.

This is a fixed version of ag/17071224, to address b/239513606.

Bug: 187702830
Test: build and check functionality
Change-Id: Ie352d07bbcfc7e0b0a1db1dbe3fd43085e0ecbb6
Merged-In: Idf1ab60878d619ee30505d71e8afe31d8b0c0ebe