- Switch setMetricsSettingListener from a listener to a 1-time query.
- Replace tryEnableGms with canUseGms. (tryEnableGms will actually be
  removed in a follow-up, since overloading implementations elsewhere
  must be removed first.)
- Fix the race condition between native AwMetricsServiceClient
  initialization and Java metrics setting query. The Java side will wait
  for native initialization before switching on metrics.
- Move the native AwMetricsServiceClient implementation to native/,
  replacing AwMetricsServiceSwitch, so it can signal the Java
  AwMetricsServiceClient when it's ready.
- Fix AwTestBase, which was incorrectly creating AwBrowserContext on the
  test thread. That was preventing us from using assertOnUiThread.
- ...which requires fixing AwStrictModeTest, which was going out of its
  way to create AwBrowserContext on the UI thread, as runOnMainSync
  doesn't allow redundant invocations.
- Some miscellaneous comment & whitespace cleanup.

BUG=676523

Review-Url: https://codereview.chromium.org/2611883002
Cr-Commit-Position: refs/heads/master@{#443776}