Add support to update the throttle time of silent updates

To improve the performance of SilentUpdateHostsideTests in the Cts,
this cl adds support to update the throttle time of silent updates
using the pm command. The `pm set-silent-updates-policy
--throttle-time TIME` could be invoked to shorten the testing waiting
time for the repeated silent updates.

Bug: 189506896
Test: atest SilentUpdateHostsideTests
Change-Id: I2f9ab58d16b7f1173f1ee45b6b842ad1f18136a4