Add 15 fps support for Android devices with missing 15 fps
camera mode.

Some latest Android devices support only 30 fps for front camera,
but HW VP8 encoder performance is not enough for 720p 30 fps
encoding. Add 15 fps support for these devices by allowing
frame drop in Android camera wrapper.

BUG=
R=tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24149004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7571 4adac7df-926f-26a2-2b94-8c16560cd09d