centralised password entry field in lockscreen

After width of password entry field was changed from
matchParent to 276dp in ag/26055623, platform scenario tests started
failing due to non-centered password entry field. This CL fixes that by
setting layout_gravity for password entry field to center

Fixes: 325656312
Test: atest PlatformScenarioTests:android.platform.test.scenario.sysui.lockscreen.PasswordBouncerSmallScreen#password_landFlagOff_portraitSmall_verticalLayout
Flag: NA
Change-Id: Ieb031a478d3eba594b906acd5129d3b2f0cc824c