Fix deadlock in RebootEscrowManager

Problem: RebootEscrowManager tries to take ActivityManager lock while
Synthetic Password Manager lock is already held.

Solution: RebootEscrowManager offload call to ActivityManager to a
handler thread.

Test: atest com.android.server.locksettings and manual testing of RoR
flow on physical device

Bug: 270006672

Change-Id: Icbc7127369cfe2eb0a6e0e554134f19b801b0274