Role: Fix resolving non-exclusive default holders

A role's default holders can be defined as a list with semicolon
separators. This commit fixes a bug where the non-split string of
default holders was used with non-exclusive roles, causing a failure of
the type:

Cannot get ApplicationInfo for default holder: com.packA;com.packB

Test: build

Change-Id: Idd986b1c49c0be5ae5f060fe436d95e5de2c01f7
Signed-off-by: Joseph Gates <jgates@squareup.com>