ApfTest: fix flaky testApfFilterRa.

testApfFilterRa is failing with probabiliy 1/10 ~ 1/15 on the following
assert: assertDrop(program, packet, lifetime/6), for lifetime values
that are multiple of 6, where 6 is the hardcoded fraction of RA lifetime
to filter in ApfFilter.java.

When the lifetime is not a multiple of 6, the remainder of 1 to 5
seconds gives enough margin so that when the APF program is simulated
the faked lifetime of the program is less than lifetime/6 away and the
packet is dropped.

However for lifetimes which are exact multiples of 6, this margin is
always 0s and that result in nondeterminism in the result. This is
consistent with the obervation that the only failed assert was for a
lifetime of 300s, the only multiple of 6.

This can be observed by detecting the age limit at which the filter
stops dropping packet oscillating between lifetime/6 and lifetime/6 + 1
for lifetimes which are multiple of 6.

This patch fixes the flakyness by freezing the flow of time in tests so
that the expected filter age threshold is consistent and stable.

Test: no failure observed in 1000 runs.
Bug: 32561414
Change-Id: I5251d047039f34b82ce8a5d20ae46563e1e0cce8