Make libjavacrypto only required for hostdex builds of conscrypt.

Workaround to prevent libjavacrypto being pulled into /system/lib rather
than APEX on target builds due to b/124476339.  There is already
a workaround in apex/Android.bp to ensure libjavacrypto is installed
in the APEX, which is the only required copy.

Bug: 123925742
Test: m && flashall; Manually verify no libjavacrypto in /system
Test: art/tools/run-libcore-tests.sh --mode=host --variant=X64
Change-Id: If8ca646755e7f9ca38dc60b67bcdb7540a394523
Merged-In: If8ca646755e7f9ca38dc60b67bcdb7540a394523