Do not throw exception on Mac#reset

OpenSSLMac was not checking whether it was initialized before
dereferencing its macKey field. This caused callers to Mac#reset()
before Mac#init() to get a NullPointerException.

Bug: https://code.google.com/p/android/issues/detail?id=58179
Change-Id: I8523983fec578079a66a04d585c1ddbe7732575e
Fix exceptions thrown in tests

Change-Id: Ia1327511d4347550111334fb858053a8cd8c3109
Some changes added to compile and run with Java 6 and Java 7.

  - correction of errors with Javac 6:
    - fields accessibility of type variable (jls 4.4).

Change-Id: I202b8767e9d4c6642e1e15e9a0570de764db0b4c
Signed-off-by: Jerome Pilliet <pilliet@univ-mlv.fr>

Conflicts:
	luni/src/main/java/java/lang/Enum.java