Properly constrain FAB to its normal bounds when tucking

Bug: 343881584
Flag: EXEMPT self-contained bugfix
Test: drag FAB to corner of the screen and release. FAB should move away from the system bars and stay visible.
Change-Id: I5b28214677ae4822d1bbf95d624a8e71a1677149
Merged-In: I5b28214677ae4822d1bbf95d624a8e71a1677149
(cherry picked from commit 195158b1)