Fix early call of UMS.removeUser causing system crash during boot.

When the removal of a user is initiated from shell command during boot,
before ActivityManager is ready, sending the user removal broadcasts throw an IllegalStateException causing FATAL Exception in system.

This CL adds a check to removeUser method to make sure ActivityManager
is ready before moving on.

Bug: 358267321
Change-Id: I51dfb73f2aada22b5cd56d734f36998cb17cfd50
Test: None
Flag: EXEMPT bugfix