Modify API behavior for PhotoPicker multi-selection flow

In an attempt to reduce implicit parameters for PhotoPicker, only
accept explicit values specified by the developer.
This helps developers to make explicit decisions on the number of items
they can handle. For example: An app should not crash if the user
selects 70 images but the app can only handle 10 at a time.

Also, add an API to indicate max limit of items that can be selected in
PhotoPicker. This is essential for apps that can handle as many images
to be selected as are allowed by PhotoPicker.

Bug: 185228105
Test: atest PickerViewModelTest
Test: atest PhotoPickerTest
Change-Id: I3ec396a35905edd9d8c5bbc7530630325bf123fd
Merged-In: I3ec396a35905edd9d8c5bbc7530630325bf123fd
(cherry picked from commit 3a3927b6205001c703b21914c4f92bc1a23e5305)
5 files changed