Making SizeSpecSource platform agnostic

Allow for customization of pip size spec on different
platforms through dagger injections instead of using a feature flag.

All the new size spec feature flag usages are removed in this CL and
the flag is about to be deprecated after.

Bug: 271468706
Test: atest WMShellUnitTests:PhoneSizeSpecSourceTest

Change-Id: I1b356a7ce23ed482682fac67d73edacddf793d75