Optimizing some icon generation code:

1) Fixing missing shadow from shortcut icons
2) Chaning all-apps icon rendering to draw background/foreground separately
   This allows us to reuse the bitmap generated for themed icons, instead of
   creating a new everytime

Bug: 248308987
Test: Verified on device
Change-Id: Ia3cbefb21a2ce676d6bb2df3d9375d61b5bed61b