Reduce buffer_hub-test flakiness

Increase the poll() timeout to 100ms to reduce test flakiness. Also
update to use glocal const kPollTimeoutMs so that we can easily tune it
in future. This also fix some compiler warnings around EXPECT_EQ.

Bug: 63909629
Bug: 68152849
Test: buffer_hub-test
Change-Id: I4e0e891125e0a1763b0f319bf47e71bc59213024