Define common test libraries in root build.gradle's ext.
Define common test libraries in root build.gradle's ext.

Change-Id: I3d14b7b88970dfe92ce2bc1e9426812615a1f08f

twitter/digits-android,metao1/digits-android,dejavu1988/digits-android,rameshvoltella/digits-android,dejavu1988/digits-android,vivekkiran/digits-android,rameshvoltella/digits-android,jaydeep17/digits-android,twitter/digits-android,metao1/digits-android,jnbt/digits-android,jaydeep17/digits-android