Add BottomSheetBehavior for picker launch animation

* Add photopicker launch animation to transition from bottom screen to
top. This happens full screen for multiselect, and half screen for
single select.
* Swiping down the PhotoPicker activity closes the activity for
multi-select, and takes the screen back to partial screen for
single-select.
* Selecting an image in half screen opens full screen.

TODO: More transition effect CL to follow to match motion specs

Bug: 185800839
Test: Manual (video attached to the bug)

Change-Id: I33b41f06b674731c7e69218ec38f439788da86ff
Merged-In: I33b41f06b674731c7e69218ec38f439788da86ff
(cherry picked from commit fcd7132bf4fec4572df3aca76ff915a4ea710316)
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index 28332ce..c911972 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -21,6 +21,7 @@
     <dimen name="dialog_space">20dp</dimen>
 
     <!-- PhotoPicker -->
+    <dimen name="picker_top_corner_radius">16dp</dimen>
     <dimen name="picker_photo_size">118dp</dimen>
     <dimen name="picker_album_size">156dp</dimen>