Remove user icon avatar from memory when unlocked

Previously, the bitmap for the user icon would stay in memory even when
the phone was unlocked. Now, the bitmap is cleared whenever the phone is
unlocked to save memory. This change only affects the user-icon shown on
the lockscreen next to the clock.

Bug: 202868702
Test: KeyguardQsUserSwitchControllerTest
Change-Id: Iec6a57adb65181fcef65489c248db77930f566bb