[Android] Implement ScreenOrientation.unlock align with spec

This is for compliance with ScreenOrientation
(https://w3c.github.io/screen-orientation).

When unlock screen orientation, it should lock the orientation
to default orientation.
Previous implementation is to set the orientation to "unspecified",
which is not fully aligned with spec.

Instead, read the configuration from the activity metadata in
AndroidManifest.xml and set the orientation accordingly.

BUG=394214

Review URL: https://codereview.chromium.org/399553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284396 0039d316-1c4b-4281-b951-d872f2087c98
(cherry picked from commit 11420f58b5bd1ea957624f482ec0c8b45245ab14)
Merge pull request #178 from wang16/master

[Android] Implement ScreenOrientation.unlock align with spec