Fixes home button showing on lockscreen in three button mode

Home button was showing because it inadvertently got caught in a change
making the home handle stay visible. This change adds a check for the
navigation mode to guard against changing nav bar button visibility when
not in gestural navigation mode.

Test: Tested locally
BUG: 148945635
FIX: 148945635
Change-Id: I15c17b771751a1e1f851b3250169f70aacfd1005