JNI optimization tweaks to Paint high-frequency methods

Bug: 22378829

Use fast-jni for getFontMetrics, drops from 35us -> 30us
Note the "heavy" part of the method, getMetricsInternal, is
already called by other fast-jni methods.

Use critical array access for getRunAdvance_* methods. This
will avoid the copy and the access is appropriately scoped
and fast enough to not significantly block the moving GC.
Improves from 88us -> 79us on short text

Change-Id: I7c1481c23f6dba3420fbcf48220f6335cf9f6d10