ART: Simplify and tighten run-test timeouts

Remove the old read-barrier special casing. Read-barrier is the
default now and performs well enough.

Use 25min as run-test default. This is derived from ASAN continuous
testing. All other continuous test cases seem to stay below 10min.
Consider a function of input values to tighten this further, e.g.,
host vs target.

Reduce the timeout in testrunner.py. Document the value and its
connection to run-test.

Bug: 140935398
Test: art/test/testrunner/testrunner.py -b --host
Change-Id: I4818f4897a675c3e14af0fb7d7257b9b52e7671c
2 files changed