gradle: set minimum sdk to 8
gradle: set minimum sdk to 8

  * There is no need to set the minimum sdk version to 15.
    Allow to use it on older apis as well

  * Hawk depends on API 8, so set the minimum sdk to 8 as well.
    Paper will build with an even lower sdk as well, just the tests
    will fail to build and API 8 is a reasonable number as it covers
    99% of the target audience.

Change-Id: I0365859041743ce078af4f02c29c86c55a64b403
Signed-off-by: Alexander Martinz <a1418a03e3f7813c1fd3f97ab13f9d68ba5a9ac2@gmail.com>

pilgr/Paper,pilgr/Paper,Haegon/Paper,pilgr/Paper,Haegon/Paper