Increase TEST_LINGER_DELAY_MS

The current timeout is short enough that lingered networks are torn down
before assertions can be run, causing flakyness in testMultipleLingering.
This CL sets the timeout to 300 in that particular test.

Verified that 300ms is enough with 500 successful runs on Sailfish.

An alternative would be to mock firing of the lingering timer, but
keeping the WakeupMessage logic realistic has already proven useful in
detecting races.

Bug: b/110673602
Test: runtest --no-hidden-api-checks frameworks-net
Change-Id: I6f9a656190bcff0b071ae2584d0f5fa74158472b