Time.parse3339 range checking and proper 'sec-frac' skip

The parse3339 JNI code doesn't properly do bounds checking on the input String.
These changes do some bounds checking to prevent a buffer underflow condition.

parse3339 should allow the fractional seconds to be optional and an arbitrary
length as specified in RFC 3339. This will scan through arbitrary precision
until it finds the timezone indicators.

Change-Id: Ie9d01d0b24163d893c58c747d37873c83b74e6c7