Previous attempt in 005a8d24ef,
reverted in 3c1d167b33.
The deduplicateStyles() function lacked a check for fosterable
positions, which caused T299767. This is now fixed.
Also added tests.
Bug: T287675
Bug: T299251
Change-Id: I0d22be9b66d26d09373cee63dd6ce52c1659e62d