Simplify Android.bp files

Remove clang: true, it's the default.

Remove target.android.shared_libs "libdl", since it's already part of
the system_shared_libs list.

Remove host_ldlibs, since -ldl and -lpthread are automatically added on
Darwin and Linux. -lrt is automatically added on Linux.

Test: mmma art
Change-Id: I18aa6aa5b49cad31caf6820b1974057ad14a2798