ANDROID: Incremental fs: wake up log pollers less often

Waking up the waiters accounts for 80+% of the total logging
time, and about 40% of overall read_single_page() with no
signature verification. By throttling it to once every 16ms
we get back all read performance, reduce the waiter's CPU
usage and still leave it enough time to pull the logs out.

Bug: 155996534
Test: adb install megacity.apk & dd from the installed apk

Signed-off-by: Yurii Zubrytskyi <zyy@google.com>
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I4a118dc226d7ca318cf099ba3e239f0120bb23c2